Can someone explain this SAT writing question to me?

Test Preparation
1

<p>its from the official SAT, 2012 - 2013.</p>

<p>Most major air pollutants can not be seen, although large amounts (of them) (concentrated in) cities (are visible) (as) smog</p>

<p>Answer is no error, but shouldn't be are as visible as smog?</p>

2

<p>No… they are in smog form…</p>

<p>It’s not a simile… they are saying air pollutants are visible in the form of smog</p>

3

<p>nope. The sentence isn’t trying to say “as visible as smog”… It’s saying “visible in the FORM of smog”. Also, an omission like that is not tested on the SAT. SAT grammar will test the omission or misplacement of conjunctions like “and, but, yet, although, because”, but not an omission like this.</p>

4

<p>thank you!</p>