Now, we have to read this carefully, you want the product of any two numbers in the set to also be a number in the set. You see easily that #1 works because an even number times an even number is an even number. #2 doesn't work because two prime numbers times each other each have those numbers as a factor. For instance, 5x7=35; well, 35 has 5 and 7 as factors. And, #3 works because a positive number times a positive number is a positive number.