藻酸盐印模材料与胶结剂的反应极为迅速，需加入缓凝剂延缓反应进

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问题单项选择题

藻酸盐印模材料与胶结剂的反应极为迅速，需加入缓凝剂延缓反应进行。常用的缓凝剂是

A．NaCO₃

B．CaSO₄

C．Ca_nAlg

D．H₃PO₄

E．ZnO

答案

参考答案：A

填空题

用适当的短语完成句子。

hear about, millions of, try doing sth. go around,

more than, send sb.(a) message(s), try to do sth.

1. I haven't seen Tom for a long time, so I ______ him ______ .

2. The earth is a planet and it ______ ______ the sun.

3. There has been life on earth for ______ years.

4. There is ______ one galaxy.

5. They have ______ send information to us.

6. —Tom, I've just heard that scientists have sent a spacecraft to Mars.

—Really?I haven't ______ this yet.

判断题

行车中，设有安全带装置的机动车，车内乘员要系好安全带。