It was along the Mississippi River _________ Mark Twain spent much of his childhood.

how

which

that

where

此处构成It was...that...强调句式，句中强调了介词短语along the Mississippi River。故选C。

It was fortunate that Jack narrowly escaped             in a traffic accident last month.

killing

being killed

to be killed

 to kill

考查非谓语动词。句意：杰克上个月在交通事故中死里逃生，真是幸运。固定表达escape doing sth.逃离做某事。此处Jack与kill之间是被动关系，应该使用动名词的被动语态，用being killed。故选B项。

Though ____ the dangers of smoking to their health, smokers still find it hard to give up smoking.

aware of

are aware of

be aware of

考查状语从句的省略：句意：虽然很多经常吸烟的人已经意识到抽烟对他们健康造成了很严重的后果，但是他们仍然发现很难戒烟。Though引导的让步状语从句是省略句，完整的句子是：though smokers are aware of the dangers of smoking to their health。当主从句的主语一致的时候，可以省略从句的主语和be动词，选B

It was last night ________ I saw the comet.

the time

when

that

which

强调句的结构是：It＋be＋强调部分＋that(who)＋主谓句。强调句的连词只有两个，that和who。当强调的部分是人，且为句子的主语时，才用“who”，其余用that。因此选C。

The student completed this experiment to make come true         Professor Joseph had said.

that

what

when

where

考查名词性从句。句意：为了使约瑟夫教授所说的成为现实，这个学生完成了这个实验。        Professor Joseph had said作make的宾语从句，该从句中，空格处在从句中作said的宾语，表示“......所说的话”，因此应用what 引导该从句。故选B项。

--- I'm afraid I can't complete the marathon next week.

---              ! You have been practicing a lot.

No problem

Go ahead

Cheer up

Good luck

考查交际用语。句意：---恐怕我下周不能完成马拉松比赛。---振作起来！你一直一直在不断练习。A.No problem没问题；B. Go  ahead请干吧；请便吧；C.Cheer up高兴起来；振作起来；D. Good luck祝您好运。由下文可知说话人担心，完不成马拉松比赛，由“你一直不断练习”可推断表示鼓励“振作起来”。故选C项。