— I think buyers ________ spend too much time going to shops.
A：needn't
B：mustn't
C：can't
D：shouldn't
往无色溶液中滴加氯化钡溶液产生不溶于稀硝酸的白色沉淀，则无色溶液中（ ）
12×30=____
546÷6=____
72×20=____
64－5=____
下列哪种植物可以监测空气的污染程度（ ）
