1) A的错误除了这里说的it指代不清楚以外,是否还有问题。把整个句子连起来看“fail to detect a condition when it is present and indicate that there is one when it is not.”,这里最后的时间条件从句“when it is not”是否不完整?因为在C中,not后面其实是省略了“present”的。在A中的这种情况下,是否也可以看成省略了“there”呢? 改成如下这个样子,对嘛? “fail to detect a condition when it is present and indicate that there is a condition when a condition is not (there).”