低碳钢拉伸试验中,经过()阶段后,材料开始产生塑性变形。
A.弹性
B.屈服
C.疲劳
D.颈缩
参考答案:B
容积式液体传动是依靠液体的压力能来进行工作的,称为().
For a long time, he hesitated whether he should tell us ____ what had happened.
A.at dusk
B.at length
C.at present
D.at stake