Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
Courtesy of the U. S. EPA. Nitrogen and sulfur oxides combine with rain to form acid rain . Public Domain.


Question: What combines with rain to form acid rain?
nitrogen and sulfur oxides
Nitrogen and sulfur oxides.