单选题 7. A mouse's immune system generally rejects proteins different from those produced by that mouse. The immune system of a pregnant mouse does not, however, reject the mouse's fetuses, although the fetal tissue a fetus produces as it develops typically contains many such proteins. Some scientists hypothesize that the placenta surrounding the fetus physically blocks the mother's immune system. But others, noting that the placenta produces IDO, an enzyme that suppresses the immune system, hypothesize that IDO plays a crucial role in protecting the fetus.
In order to choose between the two hypotheses, it would be most useful to determine which of the following? ______

A、 Whether mice sometimes produce IDO when they are not pregnant
B、 Whether the immune systems of fetal mice are capable of attacking the tissue of the mother
C、 Whether there are cases in which the immune system of a pregnant mouse rejects some fetuses but not others
D、 Whether IDO is the only substance capable of suppressing the immune system produced by mice
E、 Whether the immune systems of pregnant mice that are given a drug that inhibits IDO production subsequently reject the fetus.

【正确答案】 E

【答案解析】 E：Yes：没有IDO→排斥胎儿→IDO起到保护作用；No：没有IDO→没排斥胎儿→胎盘起到保护作用。该选项回答Yes和No分别评价了两个假说，正确。
A：和不怀孕的老鼠没关系。
B：无关选项。
C：无关选项。
D：Yes：IDO是唯一一种物质→支持了IDO保护胎儿这一观点；No：IDO不是唯一一种物质→IDO和胎盘都会保护胎儿，那到底谁起到保护作用呢?该选项回答No无法起到评价作用，不选。