became the world's largest single source of air pollution by the 1990s.
a. auto emissions
b. waste incineration
c. metal smelting
d. chemical emissions

The answer is C. Auto Emissions

Explanation:

In the 1990's cars were the new thing of the time and since they were older models they didn't and couldn't properly dispose of gasoline in a gas form that polluted the air until newer versions came out that consumed less gasoline and produced less pollution.

well let's see i know a little about the fall of the roman empore but what to say i do not have a clue of what to say. (that's 28 words) i learned about it in social studies it was quite boring really my teacher talks a lot, like a lot then gives us a bunch of work sheets not too many though.( that's 60 words)i actually almost fell asleep in class of course i really like that teacher of course she is really nice. i actually don't remember much from that class i got a b without any notes (it was an open notes test)(that's 95 words) i'm wasting time but i've gone too far to stop now i'm sorry i haven't said really any thing on the topic at all sorry not sorry haha jk.(that's 125 words ) (mike drop)