属于预防性健康行为的是（）

A.买保险

B.开车系安全带

C.企业对"三废"的处理

D.适量饮酒

E.适度锻炼

We meant to finish the work by dark, but we were so tired that we could not ______.

A．hold on

B．last on

C．carry on

D．keep to