What events could have created the increase on the acid precipitation between 1955-1990?

Question

What events could have created the increase on the acid precipitation between 1955-1990?

Answer 1

Acid rain is caused by large amounts of nitric oxides being dissolved in the rain, nitric oxides are often released by polluting factories so the increase is probably because there were more factories releasing more pollutants into the atmosphere.

Answer 2

-increasing industrialization of large, third world countries

-cars becoming increasingly common