复合题  Directions: For passages 1 to 4, each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A., B., C. and D. Decide on the best choice and write the corresponding letter on the Answer Sheet.

Passage 4

People have been donating blood since the early twentieth century to help accident victims and patients undergoing surgical procedures. Usually a pint of whole blood is donated, and it is divided into platelets, white blood cells, and red blood cells. People can donate blood (for red blood cells) about once every two weeks.

Transfusing the blood frown the donor to the recipient is straightforward. It involves taking the blood from a donor’s arm vein by means of a hypodermic syringe. The blood flows through a plastic tube to a collection bag or bottle that contains sodium citrate, which prevents the blood from clotting.

When the blood is given to a patient, a plastic tube and hypodermic needle are connected to the recipient’s arm. The blood flows down from the container by gravity. This is a slow process and may last as long as 2 hours to complete the infusion of blood into the recipient. The patient is protected from being infected during the transfusion. Only sterile containers, tubing, and needles are used, and this helps ensure that transfused or stored blood is not exposed to disease-causing bacteria.

Negative reactions to transfusions are not unusual. The recipient may suffer an allergic reaction or be sensitive to donor leukocytes. Some may suffer from an undetected red-cell incompatibility. Unexplained reactions are also fairly common. Although they are rare, other causes of such negative reactions include contaminated blood, air bubbles in the blood, overloading of the circulatory system through administration of excess blood, or sensitivity to donor plasma or platelets.

Today, hospitals and blood banks go to great lengths to screen all blood donors and their blood. All donated blood is routinely and rigorously tested for diseases, such as HIV (which causes AIDS), hepatitis B, and syphilis. When the recipient is a newborn or an infant, the blood is usually irradiated to eliminate harmful elements. Donated blood is washed, and the white blood cells and platelets are removed. 

单选题 According to the passage, how often can people donate blood for red blood cells?
【正确答案】 D
【答案解析】事实细节题。 文章第一段最后一句“People can donate blood (for red blood cells) about once every two weeks”， 指出人们可以每两周献一次血（红细胞） 。 由此可以得知D符合题意。
单选题 Which sentence in Para. 2 explains how clotting is prevented in the blood container?
【正确答案】 C
【答案解析】事实细节题。 文章第二段最后一句“The blood flows through a plastic tube to a collection bag or bottle that contains sodium citrate, which prevents the blood from clotting”， 血液通过一条塑料管流到装有柠檬酸钠的收集袋或瓶中， 这样可以防止血液凝固。 选项C符合题意。
单选题 What answer choice is closest in the meaning to the word “undetected” in Para. 4?
【正确答案】 C
【答案解析】词汇题。 undetected未检测到的； 未被发现的。 not found未被发现的。 not captured未被捕捉到的。not illustrated未被阐明的。
单选题 All of the following are mentioned as potential negative reactions to transfusions EXCEPT _____.
【正确答案】 C
【答案解析】事实细节题。 根据文章第三段可知negative reactions（副作用） 包括： allergic reaction（过敏反应） ， sensitive to donor leukocytes（对供体白 细胞过敏） 和red-cell incompatibility（红细胞不亲和性） 。 因此， A、 B和D项符合原文。 而C项“bubbles in the air（空气中的气泡） ”是造成副作用的原因， 而不是结果，因此选C。
单选题 It can be inferred from the passage about blood transfused to infants and newborns that _____.
【正确答案】 B
【答案解析】事实细节题。 文章最后一段倒数第二句“When the recipient is a newborn or an infant, the blood is usually irradiated to eliminate harmful elements”， 当受捐者是婴儿时， 血液要经过辐射去掉一些有害元素。 因此B项“用辐射能源辐射”符合题意。