—Your sister’s plan is not quite practical.Why not try to persuade her to give it up?

—My sister is very stubborn，so it is________to try to change her mind.

A．by chance

B．in vain

C．on purpose

D．for sure

下列不属于高分子化合物的是。

A．蛋白质

B．乙醇

C．淀粉

D．聚乙烯