2. 外语
  3. Throughout the world there is a shortage of human organs for transplants. In Britain, for example, 6000 people are waiting for o

Throughout the world there is a shortage of human organs for transplants. In Britain, for example, 6000 people are waiting for o

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问题     Throughout the world there is a shortage of human organs for transplants. In Britain, for example, 6000 people are waiting for organs ——5000 for kidneys, and the (51) for hearts, lungs and livers. (52) , only about 1750 kidney, 500 heart and lung, and 650 liver transplants are (53) each year. And the waiting (54) increases at five percent a year. In the United States only half of the 30000 in need of organs (55) them.
    Hence the great interest in animal-to-human transplants, known as xenografts or xenotransplants (from the Greek work xenos, meaning strange of foreign) (56) most scientists believe is the only long-term solution to the organ (57) . There have been attempts at xenotransplants (58) the beginning of this century but neither has been successful; the longest (59) was a 20-day-old baby called Fae, who in 1984 was (60) the heart of a baboon.
    The main (61) with any organ transplant is that the immune system of the patient receiving the organ sees the transplant organ (62) an invader. The immune system therefore launches a massive attack on the invader, activating enzymes knows as complement, which attack the (63) body, eventually killing the patient as well. This rejection has been (64) when transplanting human organs between humans by the close matching of tissue and the long-term use of drugs known as immuno-suppressants, first introduced just over ten years ago. With xenotranplants, however, rejection is even more (65) ; a normal pig’s heart, for example, transfuses with human blood can be destroyed in 15 minutes.
选项 A、since
B、at
C、in
D、after
答案A
解析 since引导的时间状语从句，主句应用现在完成时。
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