so we want to know the value of (x - y)^4, 1) the product of x and y is 7, since this doesn't tell much except x * y = 7, and since x and y could be non-integers(ie: if x = 3.5 and y = 2, then the answer is different from when x = 7 and y = 1), we don't know the answer, thus 1) by itself is not sufficient 2) x and y are integers, by simply knowing that x and y are integers, we can not conclude what the value of (x - y)^4 is, for x and y could be any integer, thus 2) by itself is not sufficient 1) and 2) together, now we know that x * y = 7 and x and y are integers, thus, x and y could only be either (7, 1), (1, 7), (-7, -1), (-1, -7), and when x and y are any of the above pair, the value of (x - y)^4 = 6^4, thus, 1) and 2) together are sufficient, thus the answer is C | 
