One strategy for identifying necessary assumptions is the "negation rule." You take all the answer choices one by one, make them false (negate them) and see which one invalidates the argument -- makes it impossible to reach the conclusion from the stated premises. This rule works well when applied correctly, but can sometimes be tricky -- search the forum for "necessary assumption" and you should find some interesting threads.
That is an awesome tip! I'm about to do an LR section, here's hoping it helps me