when 来引导。

　　答案：when

　　3．When I arrived, Bryan took me to see the house________I would be staying.

　　解析：此处是定语从句，从句中缺少地点状语，故用 where。

　　答案：where

　　4．There is no simple answer, ________is often the case in science.

　　解析：as is often the case 表示"这是常有的事"，as 指代前面主句的内容。

　　答案：as

　　5．________is often the case with children, Amy was better by the time the doctor arrived.

　　解析：句意：对于孩子来说经常会有这样的情况发生，当医生到达的时候，艾米好多了。根据句意并分析句子成分可知，空处引导一个非限制性定语从句，指代主句，又因位于句首，所以用 as，在从句中做主语。as is often the case with...意为"......的情况经常发生"。

　　答案：As

　　6．The children, all of________ had played the whole day long, were worn out.

解析：先行词为 children，指人，且关系代词在非限制性定语