为什么是B呢？感觉B是support结论，不是weaken呀？？？请路过的给解解疑惑！

Mice that have been given morphine are very likely to develop blood poisoning because bacteria that normally reside in the intestine typically respond to morphine by migrating into the bloodstream.  However, when mice are given both morphine and the new drug naltrexone, blood poisoning is much less frequent, although it does still occur.  These results provide support for researchers’ prediction that naltrexone will turn out to be toxic to certain types of bacteria.

Which of the following, if discovered to be true, would most seriously weaken the support for the researchers’ prediction?

A.      After being administered to mice, naltrexone does not pass from the bloodstream into the intestine.

B.       Naltrexone inhibits morphine from triggering the migration of intestinal bacteria into the bloodstream.

C.      Mice that have been given naltrexone but not morphine have no greater risk of developing blood poisoning than do mice that have not been given either substance.

D.      The increased risk of blood poisoning is not the only harmful effect on mice of being given morphine.

E.       Conditions other than the presence of intestinal bacteria in the bloodstream can cause blood poisoning in mice.

请注意黑体部分：

结论是：These results provide support for researchers’ prediction that naltrexone will turn out to be toxic to certain types of bacteria.

重点在于说N对于细菌有毒害作用。潜台词是说，因为有M存在，细菌还是进到老鼠的血液了，只是因为N毒害了细菌，所以老鼠不生病了。

Naltrexone inhibits morphine from triggering the migration of intestinal bacteria into the bloodstream.

而B选项说的是N 阻止M让细菌进入血液。题目一开头说为什么给老鼠M后会毒害他们的血液呢？because bacteria that normally reside in the intestine typically respond to morphine by migrating into the bloodstream，M让细菌进入血液。

这样，B选项实际构成的是他因削弱。因为B让细菌根本进入不到血液中，而不是因为B对细菌有毒害作用。