? Small, freshwater zebra mollusks have been invading America"s freshwater lakes and rivers for more than fifteen years. The shellfish are native to Russia, the Balkans, and Poland and were originally found in the areas of the Ural River and the Black, Caspian, and Azov Seas in the 1700s. By the late 1700s and early 1800s, they had spread to many parts of Europe because of the numerous canals. In 1824 these mussels were discovered in Great Britain. Today, they have infested most of Western Europe as well as the Scandinavian countries, Ireland and Italy. Zebra mollusks were transported to North America in 1988 in the ballast water of a transatlantic freighter and were discharged into Lake St. Clair, an inland lake in Michigan which connects Lake Erie and Lake Huron. Within four years the mollusks had infested one lake. Within ten years, more than one hundred lakes had become home for these invaders. Today, these invasive mussels have established themselves in all of the Great Lakes, other inland lakes in Michigan, and the basins of most navigable rivers in the eastern United States including the Mississippi, Tennessee, Ohio, and Hudson Rivers. Their presence has also recently been detected in the Missouri River basin.
? Zebra mollusks, which get their name from the striped pattern of their shells, are prolific breeders. Females can produce between 30,000 and one million eggs each year. During their planktonic larval stage, young zebra mollusks are the size of a human hair and appear invisible to the human eye and the tiny shellfish can live and feed in many different aquatic habitats, and they can move easily and swiftly in water currents and can drift many miles before settling. Adult zebra mollusks live about five years and can survive in fresh water at levels deeper than twenty feet. Once the mollusks reach their adult stage they attach themselves to hard objects in water. Boats, buoys, piers, and fishing gear can be their victims as well as clams, crayfish, turtles and other creatures living in the water. Zebra mollusks also attach themselves to each other to form huge colonies.
? Environmentally, zebra mollusks can be a threat. When they attach themselves to watercraft, the mollusks can cling to the hull or motor or any part of the boat which is immersed in the water. As a result, boats can overheat, have increased drag, and show damaged engine cooling systems. Piers, navigational buoys, and other structures can sink under the weight of the attached mollusks or suffer deterioration and long term damage from the attachment of the shellfish. Many municipalities have been required to redesign the intake systems for their city water supplies because mollusks have clogged the intake pipelines. Other communities have resorted to chemicals to treat and prevent zebra mollusk problems. The mollusks can threaten native wildlife and marine life and upset ecosystems. Marine life is also severely affected by zebra mollusks when the mollusks attach themselves to slow moving animals such as turtles and crayfish or to clams and mussels which are found naturally in fresh water lakes. When the zebra mollusks attach themselves to an object in large numbers, the mollusks are said to be "colonizing." Colonizing interferes with the object"s ability to feed itself and its growth, movement, respiration, and reproduction abilities.
[■] Despite problems caused by the mollusks, they also provide benefits to polluted bodies of water. [■] As they filter water, they digest animals and algae that provide food for other fish and animals. [■] The benefit of the filtering process, however, is that water clarity in infested lakes and rivers increases dramatically. These tiny shellfish also provide food for many native birds and fish but they cannot be eaten in amounts great enough to control the mollusk population. [■] Zebra mollusks help provide more healthy environments for aquatic plants, and in some inland lakes and rivers the smallmouth bass population has increased because the additional aquatic plants have provided feeding and nursery areas for young fish.
? Environmental agencies, local communities, and inland lake associations are working diligently to protect people and waters from the threat of the zebra mollusks. Once the mollusks become established in bodies of water, eradication is difficult, if not impossible, because the creatures seem to be oblivious to all but the harshest chemicals—chemicals which can kill other plant and wild life. Because zebra mollusks can survive out of the water for as long as five days in cool, humid climates, boaters, fishing enthusiasts, and all lake lovers need to learn how not to transmit the invaders, in larvae and adult form, between bodies of water.

freighter: a large ship or airplane that is designed for carrying freight
breeder: people who breed animals or plants
buoy: a floating object that is used to show ships and boats where they can go and to warn them of danger

单选题 Based on the information in paragraph 1, which of the following best explains the term zebra mollusks?

单选题 The word ballast in the passage is closest in meaning to

单选题 Based upon the information in paragraph 2 and paragraph 3, what can be inferred about the mollusks mentioned in paragraph 1?

单选题 According to paragraph 5, which is true about zebra mollusks?

单选题 In paragraph 2, the author states that the water currents

单选题 According to paragraph 1, what can be inferred about the spread of zebra mollusks?

单选题 What is the problem in eradicating the zebra mollusks according to the last paragraph?

单选题 Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
Within ten years, more than one hundred lakes had become home for these invaders.

单选题 In paragraph 3, why does the author explain that mollusks can cling to boats?

单选题 The word discharged in the passage is closest in meaning to

单选题 The word dramatically in the passage is closest in meaning to

单选题 Which of the following statements more accurately reflects the author"s opinion about environmental problems associated with zebra mollusks?

填空题 Look at the four squares [■] that indicate where the following sentence could be added to the passage.
Because they are filter feeders, zebra mollusks can filter about a quart or liter of water each day while feeding on algae.
Where would the sentence best fit?
Click on a square [■] to add the sentence to the passage.

多选题 Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they may express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Drag your answer choices to the spaces where they belong. To remove an answer choice, click on it. To review the passage, click on View Text:
Zebra mollusks have arrived in huge numbers to infest American lakes and waterways.
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Answer Choices: