2. 外语
  3. For some birds the sense of smell appears to play a role in navigation, since pigeons with surgically removed olfactory nerves w

For some birds the sense of smell appears to play a role in navigation, since pigeons with surgically removed olfactory nerves w

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问题 For some birds the sense of smell appears to play a role in navigation, since pigeons with surgically removed olfactory nerves were found to have increased difficulties in homing.
选项 A、were found to have increased difficulties
B、have been found to have increased difficulty
C、were found to have increasing difficulty
D、had been found to have increased difficulties
E、have been found to have increasing difficulties
答案B
解析 A、C、D中过去时或过去完成时与主句的一般现在时态是不对应的。句子的前后是因果关系(由since引导的原因从句)，但当把过去时或过去完成时代入句中，就会发现前后的因果关系很模糊，甚至不存在。Why?因为过去的时态仅表明过去此发现是客观的，并不表明这种发现至今仍客观存在。因此主句所下的论断appears to play a role in navigation就是缺乏事实根据的。B、E中用现在完成时使得前后时态一致，逻辑关系准确。而此处，difficulties的增加是由于被去掉嗅觉系统后造成的，因而increase是被动动作，而非主动动作，所以选B。还记得我们在第一章第一个例子时说的：在选择用increased还是increasing作限定词，必定是increased正确。
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