题目

There__________ a lot of trees here, but now it has become a desert. A . is B . have C . used to be D . used to being 答案：C20．下列各组物质，按化合物、单质、混合物的顺序排列的是（　　）A．干冰、铁、氯化氢B．生石灰、白磷、纯碱C．烧碱、液态氧、碘酒D．空气、氮气、胆矾（CuSO4•5H2O）