下列熟悉的物质中属于纯净物的是[ ]

A．汽水

B．酒精

C．食醋

D．碘酒

— Now the air in our city is _____ than it used to be.

— I agree. I think something must be done to stop it.[ ]

A. much better

B. even worse

C. very good

D. rather bad