GMAT Club Official Solution:If x and y are integers and (x - 1)(y + 3)(xy + 3) = 0, which of the following must be true?
I. If x = -1, |y| is a prime number
II. If |y| is not a prime number, x is not a prime number
III. If x is a prime number, y/x is not an integerA. I only
B. II only
C. III only
D. I and II only
E. I, II, and III
Since (x - 1)(y + 3)(xy + 3) = 0,
at least one of the following must be true:
• x = 1
• y = -3
• xy = -3. Since x and y are integers, xy = -3 gives:
x = 3 and y = -1
x = -3 and y = 1
x = 1 and y = -3
x = -1 and y = 3
So the equation can be satisfied if
at least one of the following is true:
1. x = 1
2. y = -3
3. x = 3 and y = -1
4. x = -3 and y = 1
5. x = 1 and y = -3
6. x = -1 and y = 3
Now check the statements.
I. If x = -1, |y| is a prime number
If x = -1, case 1, 3, 4, and 5 are impossible. So either y = -3 from case 2, or x = -1 and y = 3 from case 6. In either case, |y| = 3, which is prime.
Therefore, statement I must be true.
II. If |y| is not a prime number, x is not a prime number
If |y| is not a prime number, cases 2, 5, and 6 are impossible. However, case 3, x = 3 and y = -1, is still possible. Here, |y| = 1, which is not prime, but x = 3, which is prime.
Therefore, statement II is not necessarily true.
III. If x is a prime number, y/x is not an integer
If x is a prime number, cases 1, 4, and 6 are impossible, since x is not prime in those cases. However, case 2, y = -3, is still possible. And if y = -3, x can be any prime. For example, if x = 3 and y = -3, then y/x = -3/3 = -1, which is an integer.
Therefore, statement III is not necessarily true.
Answer: A.