Look! David is helping his mother _______ the room.
A:is cleaning
B:cleaning
C:cleans
D:clean
含羞草受到碰触时.展开的叶片会合拢,这一事实说明( )
如图为某大洋局部示意图,若AB线是30°纬线,则这个海域位于( )
四瓶失去标签的溶液分别为①NaCl ②CuSO4③BaCl2④H2SO4不用任何试剂将它们一一检验出来的正确顺序是( )