【正确答案】The professor talks about two ways animals ensure humidity inside the eggs they lay. The first way is to lay eggs directly in water. For example, frogs lay their eggs in the water in order for the eggs to absorb enough moisture. This saves the parents a lot of trouble, and once the eggs are safely laid, they depart.
However, some animals cannot lay eggs in water, and they need to find another way out to ensure the humidity of their eggs. For example, snakes usually lay eggs in a small bag with enough moisture inside. So the snakes can lay eggs even in dry areas.

【答案解析】[听力原文]
W: Today, I'd like to touch on some species of egg-laying animals. Remember now, all egg- laying animals must ensure that their eggs have sufficient moisture to survive. At this point, dry out means death to the unborn young. There are two main ways animals keep the egg moist.
The simplest way, of course, is to lay the eggs directly in water. Frogs, for example, adopt this method. The eggs can absorb water directly from the environment, and the frog parents need not trouble themselves further.
More interesting, another method to ensure the humidity of eggs is used by snakes. Most snakes, after all, live on dry land and can't rely on water for moisture. Such snakes simply encase the needed moisture inside of a bag. Then the eggs are laid in the bag. The young grows inside the moist bag and hatches from the egg just like a bird. This is most common among snakes living far away from wet environments. In this way, you see, the snake avoids the danger of the too-dry environment.

Alternative Wording(替换表达)
Synonyms (a)ensure—guarantee (b)lay—deposit
(c)enough—sufficient (d)dry—parched
Paraphrases 1. Frogs ensure the moisture supply to their eggs by depositing them directly in the water.
2. This permits the snake to lay eggs even in parched areas.