多次强制循环的循环动力来自(　　　)

A．回路中工质重度差

B．给水泵

C．循环泵

D．循环泵与重度差的联合作用

Here      you new shoes. Please put        .[ ]

A. is, them on

B. are, they on

C. are, them on

D. are, on them