crackgmat007 wrote:

(2) the clauses are really short: i ordered steak and she ordered shrimp.

Wow!! I would have treated the above example as ungrammetical. Thanks for clarifying Ron.

So, if two clauses are really long with modifiers etc, we do need a comma. Correct?

i would generally say "yes".
however, from the tone of your post, it seems as though you're looking for a RULE that is strong enough to serve as the actual basis for making eliminations; thi sis not the sort of topic on which i would propound such a rule, since the difference is mostly stylistic rather than grammatical.

in other words:
this is not the sort of thing upon which you should try to eliminate answer choices, unless you literallly cannot find ANYTHING else that you don't like about the remaining choices.