骨、骨胳肌和关节在运动中所起的作用相当于（　　）

A．杠杆、支点、动力

B．支点、杠杆、动力

C．动力、杠杆、支点

D．杠杆、动力、支点

—We’ll visit the Summer Palace_____ there is a heavy rain tomorrow.

— OK! Boating on the lake must be great fun.

A．if

B．as soon as

C．unless

D．since