back next Monday morning.下个星期一上午回来" 是暗示语,说明还没有去Hong Kong故用现在进行时表将来，答案为B 。⑵下文的Then I was asked to go to a conference in Beijing 给了暗示， 说明当时在Hong Kong仅仅呆了两天 ，这是一个过去的事实与for 短语无关。故选C。w.w.w.302edu.c.o.m

　　5.⑴ It was already six o'clock ____________we arrived home yesterday .

　　⑵It was at six o'clock ___________ we arrived home yesterday .

　　A. that B. since C. when D. on which

　　解析：解答本题的关键看是否是强调句型。如果把强调句型的结构去掉，句子的结构完整就是强调句型;否则就是时间状语从句 。⑴去掉it was -----that 句子结构不完整，故选C。 when 引导的是一个时间状语从句， it 指时间。 ⑵去掉it was -----that 句子结构完整，故选 A 。 that 引导的是强调句型， 强调的是时间状语at six o'clock .

　　6. ⑴It isn't that long __________I've gone , but it seems so long .

　　A. since B. when C. as D. that

　　 ⑵It was not long ____________ he sensed the danger of the position.

　　A. when B. until C. since D. before

　　解析：⑴ 的答案为 D , 这是一个强调句型 ， 强调的是that long 指时间。

　　⑵的答案为D 。 本题考察是：It wasn't long before -----句型"不久就------"。 句意为：不久他就意识到了这个位置的危险。

　　7. ⑴ Is this research center __________you visited?

　　⑵Is this the research center ________you worked ?

　　A. that B. the one C. which D. where

　　解析：解答本题先把这两个疑问句变成陈述句。w.w.w.302edu.c.o.m

　　⑴变成：This research center is __________you visited.

　　⑵变成: This is the research center __________you worked .由上可知，句1后面的定语从句无先行词，必须加先行词，故用the one代替 , 而它后面的定语从句的关系词在从句中作宾语，可用that 、which 或省略不填 ， 故选B。句2 是一个完整的定语从句，其先行词是the research center ,从句中的work 是 不及物动词，关系词在从句中作work的状语， 故选 D。

8.⑴I can think of many cases __________students obviously knew a lot of English words and expressions but couldn't write a good essay .