在孔与轴的配合中，孔的尺寸减去相配合的轴的尺寸，其偏差为（）时，即形成间隙配合。
A.负值
B.正值
C.零
D.正值或负值
参考答案：B
生产实践中常用的选种方法主要有哪四种？
The monitor theory, which is very popular among foreign language teachers in______, was put forward by Stephen Krashen in the late 1970s.
A．America
B．Britain
C．China
D．France
