沙化土地实行（）、综合治理、合理利用是我国开展防沙治沙工作的基本要求。
A.编制规划
B.分类保护
C.土地监测
D.限制扩散
参考答案：B
Please tell Tom not to leave the classroom unless he ______ that all the lights ______.
A．makes sure; are turned off
B．makes sure; will be turned off
C．is made sure; are turned off
D．will make sure; will be turned off
重550牛的工作人员乘热气球匀速下降．设备重200N，已知热气球自重为400牛，已知空气阻力为50N，则热气球受到的升力是______N．要使气球匀速上升，则需扔掉______N的设备．
