Necessary assumption. Use negation. I said these two sentences hundreds of times here. If you master the technique of negation, you should be able to solve the necessary assumption problems easily and pass GMAT with flying colors.

When you negate an answer, just negate it. Try not to turn it into its opposite. Also, negate either the verb or the quantity, but not both.

To negate an if-then statement, negate ONLY the then clause. So "if A, then B" would become "if A, then not B"

Quantitites                                 |  Negation
all, any, each, every, always          |  not all, some are not
most, majority                              |  not most
usually, likely                                 |  not usually, not likely
some, many, few                          |  none
sometimes, often, rarely                |  never
only one, only once                      |  more than one, none (That's right. Not one = none or more than one)
exactly two                                   |  one, more than two, none (similar to the reason listed above)
no, none                                       |  some


Verbs                                            | Negation
will                                                  |  might, might not (Another case of paradox!)
can, could                                      |  cannot
there is, there are                           |  there is not, there are not