我觉得用排除法可能更好,
根据主谓一致,有helps的干掉
E between by 很别扭,干掉
至于B选项感觉意思变了,说的是两耳有区别,且 hearing a given sound帮助了听者决定.....
A也就是原句的意思是两耳在听到同一声音的不同帮助了听者决定.....所以B干掉
另外HELP
正确: He HELPS RAKE the leaves.
He HELPS TO RAKE the leaves.
He HELPS me RAKE the leaves.
He HELPS me TO RAKE the leaves.
His HELP IN RAKING the leaves has been welcome.
错误:He HELPS me IN RAKING the leaves.
I need him AS HELP TO RAKE the leaves.
以上是HELP的用法,应该是只有做名词的时候后面跟doing
然后不建议死记硬背语法点,一般的题通过排除法都可以干掉