20世纪60年代以来，不少国家的气象学家用云的催化方法在干旱季节做增雨试验，这种试验叫人工降雨，科学家使用的是()。

You have made a few mistakes in your composition but you have done well.

—Shall we take a taxi there since time is limited?

— ________. It should be hard to get one during the rush hour.

I think it must have rained,__________?

下列不属于酸雨形成的主要原因是()