填空题。

1.  The question is so hard that _______students can answer it. (用a few或few填空)

2. —Is the new rule working?

    —Yes, ______books are stolen (被偷). (用ra few或few填空)

3.  Don't worry,   we still have ______minutes left. (用a few或few填空)

4. There are so ______trees on the hill.  The birds can't even find a place as thei home.

(用a few或few填空)

下列物质用途不合理的是

A．用氮气作粮食的保护气

B．用熟石灰作为人体的补钙剂

C．用纯碱除去面粉发酵产生的酸

D．硝酸钾在农业上用作复合肥