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All possible m are: 3, 5, 7, 9, 11, 13, 15, 17, 19
a. m is not divisible by 3, thus delete 3, 9 and 15. The rest of the odd numbers are all prime numbers which means they are all divisible by exactly 2 numbers: 1 and itself. Sufficient
b. m is not divisible by 5, thus delete 15. Except 9, all of the rest are prime numbers, which are divisible by two numbers: 1 and itself. 9 is divisible by three numbers: 3, 1 and 9. Thus, b is insufficient