恒定浓度在塔内变化，对萃取精馏有何影响？

–The classroom is_____ clean_____ it was yesterday.

--Sorry, I forgot to clean it.

A．as, as

B．so, as

C．not so, as

D．more, than

某物体运动的速度图象如图，根据图象可知

A．物体是从静止开始运动的

B．物体位移方向一直没有改变

C．物体运动方向一直没有改变

D．物体在运动过程中受到的合力一直没有改变

China not only will endeavor to curb its population growth, but will also upgrade the education of its citizens.

A．will not only ... but also will

B．will not only ... but also will

C．will not only ... but also

D．not only ... but will also

如图所示，ABCDEF为一个正六边形，现以A为顶点向其它各定点作矢量线段，并用它们依次表示F₁….F₅等各力，若其中F₃的大小恰为5N，那么这五个力的合力为多大，方向如何？

A．15N与F₃同向

B．30N与F₃同向

C．15N与F₁同向

D．30N与F₂同向

维护和增进老年人心理健康的原则有（）、（）、（）。