What they gave us is ______ help; they g

千里之外拒人于（）

A．问到底打破沙锅

B．今朝有酒今朝醉

C．♀海而皆准♀放之四

D．♂前亏♂好汉不吃眼

由五十亿、七亿和六千组成的数是______，把它精确到亿位约是______．

正常小儿后囟闭合最迟在（）。

A.2个月

B.4个月

C.6个月

D.12个月

E.18个月

Most whales must come up for air after several minutes, _______ a few can stay under water for an hour or more.

A．while

B．or

C．for

D．so

.

________ being very convenient to shoppers, plastic bags also cause serious white pollution.

A．Because

B．Since

C．While

D．If