Nature reserves are very important for preserving many animals and plants, which would _______ run the risk of becoming extinct.

A．otherwise

B．therefore

C．though

D．instead

下列说法中正确的是（）

A.相互接触的物体之间一定有弹力作用

B.不接触的物体之间也可能有弹力作用

C.压力和支持力的方向都垂直物体的接触面

D.压力和支持力的方向都平行于物体的接触面