有读者问： 有这样一道选择题：
(1)The road was narrow, _____made it difficult to drive.
A. that B. which C. as D. this
这里为何只能选B而不能选C?
在关系代词as用作主语时，其动词一般用be或其他连系动词，如：
(2)She was late for school, as was usual with her.
(3)He has married again, as seemed natural.
上述两句中的as亦可代之以which（但as表为人所熟知的信息，which则表新的信息）。但关系代词which用作主语时，其后则可用行为动词，上述句（1）之所以须用which（后接行为动词made），其原因就在此。再如：
(4)He has married again, which delighted us.
