## 语法题目 ##
Although beavers rarely remain submerged for more than two minutes, they can stay underwater ____ fifteen minutes before having to surface for air.
A. as long
B. as long as
C. so long
D. so long that
## 题目解析 ##
答案解析：句子主谓结构完整，空格作状语。so long that 后面连接句子，as long 和 so long 不是常用结构，选择 B，as long as 长达……。
空格前为主句，且主谓结构完整，根据句意，可判断空格及其后面的内容充当句子状语，表示「长达……」。空格后的内容是一个名词短语 fifteen minutes，所以空格出需要填入的是介词，而非连词。B 选项和 D 选项都可以表示「长达……」，但 D 选项 that 后要接句子，不符合空格后的内容，故排除，所以 B 选项为正确答案。
## 知识点梳理 ##
常见介词：at, by, for, from, in, near, of, off, on, during, since, before, after, until; inside, out of, within, without, outside, throughout; as to, until after, except for; at the cost of, at the mercy of, by means of, in place of, by favor of, in spite of, in/with reference to, in/with regard with
① I have lived here <u>for a year</u>.
② I'll wait for you <u>till 10 o'clock</u>.
③ I <u>was familiar with</u> nobody <u>at the meeting</u>.
④ Swimming <u>is good for your health</u>.
⑤ We can see <u>with our eyes</u> and write <u>with our hands</u>.
⑥ We succeeded <u>by cooperating</u> <u>with them</u>.
⑦ She seems to know the solution <u>to the problem</u>.
常见从属连词：when, while, as, before, after, until, till, since, if, unless, so that, because, although 等
常见并列连词：and, or, but, yet, for, either…or, both…and, not only…but also, as well as, neither…nor 等
① <u>When he comes</u> I'll tell him about it.
② I wasn't long <u>before he told us about it</u>.
③ We <u>danced and danced</u> <u>until they all joined in</u>.
④ He was worried <u>because he hadn't had any letter from her</u>.
⑤ <u>Although everyone played well</u>, we lost the game.
⑥ We were <u>singing and dancing</u> all evening.
⑦ She has been learning English for six years, <u>but she doesn't speak it very well</u>.
⑧ The days were short, <u>for it was now December</u>.