更换钢球磨煤机钢甲后应如何紧固钢甲螺丝？

Everyone ______ be polite to the old people.

A．may

B．should

C．can

D．could

Oh, look at you---you are trembling! You must feel cold now. In fact, you __________ in person. You can contact us by phone next time.

A．didn’t need to come

B．needn’t come

C．needn’t have come

D．don’t need come

个体与社会相互独立性的表现。

直齿锥齿轮强度计算时，是以（）为计算依据的。

A、大端当量直齿锥齿轮

B、齿宽中点处的直齿圆柱齿轮

C、齿宽中点处的当量直齿圆柱齿轮

D、小端当量直齿锥齿轮