-- Mr. White, will you please teach me how to perform pair skating today?

--         . You have to practise single skating well first.

A．Two heads are better than one.

B．Think twice before you do.

C．Learn to walk before you run.

D．One today is worth two tomorrows.

4-3规格的绞辘，其动、定滑轮数分别为（）

A.4，3；

B.3，3：

C.4，4；

D.3，4。