如图12—4所示，电源电压恒定， 当S接a时，电流表A₁与A₂的示数之比为

3：5；当S接b时，电流表A₁与A₂的示数之比为2：3，则R₂与R₃的电阻之比为(    )

图12—4

A．9；10

B．4；3

C．3；4

D．5：2

Late in the afternoon, the boys________their tent in the middle of a field. [      ]

A. put back  

B. put on

C. put up  

D. put out