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 is said that railway station is now         large as What it was.

as twice

twice as

twice much

much twice

考查倍数表达法。句意：据说火车站现在是原来的两倍大。倍数的基本结构一般为“A+倍数+as+形容词或副词的原级+as+B”，表示“A正好是B的多少倍”。联系下文large as，此处用twice as large as 表示“是.....的两倍大”。故选B项。

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。

Tom looks so pale today. He ______ ill.

must be

had to be

should be

shall be

[翻译]汤姆看起来脸色不好，他一定生病了。[解析] A 试题解析：情态动词must表示推测。题目中表示肯定的推测。