下列物质中属于纯净物的是 [ ]
A. 空气
B. 粗盐
C. 生铁
D. 纯碱
答案：D
界限含水率的测定可评价______。
A．各种土的状态
B．黏性土的状态
C．土的塑性范围的大小
D．黏性土的结构
26. It is difficult to do some of the experiments ________ in our physics textbooks.
A．to describe
B．describing
C．described
D．being described
