地球自转线速度最大的地方是：（   ）

A．南北极点

B．南北极圈

C．南北回归线

D．赤道

—Our environment is getting worse than before.

—You're right. But people have done more and more

    useful things to protect (保护) the earth since Earth Day _____. [ ]

A. is started

B. was started

C. has started