题目

一个半径比地球大2倍，质量是地球36倍的行星，它表面的重力加速度是地球表面的重力加速度的（    ）A.6倍          B.18倍          C.4倍               D.13.5倍 答案：C解析：在地球表面处：G=mg在某星球表面处：G=mg′故=4即正确选项为C.They have discussed for several years of drafting a new law to control the pollution, but when it will be        is still unknown.     [     ]A．put into operation         B．put in effect    C．put to use                D．put to effect