为保证环状给水管网的水源，向环状管网输水的输水管不应少于()条。
A、一
B、二
C、三
D、四
参考答案：B
_____ work did they have to do that they had no time to take a rest at that time.[ ]
A. Too much
B. So much
C. Too many
D. Such little
常用的耐火材料有耐火砌体材料、耐火水泥和（）。
A.耐火混凝土
B.耐火石棉
C.玻璃纤维
D.蛭石
