Official Solution:If \(x\) and \(y\) are positive integers, and \(xy\) is divisible by 4, which of the following must be true? A. If \(x\) is even, then \(y\) is odd.
B. If \(x\) is odd, then \(y\) is a multiple of 4.
C. If \(x+y\) is odd, then \(\frac{y}{x}\) is not an integer.
D. If \(x+y\) is even, then \(\frac{x}{y}\) is an integer.
E. \(x^y\) is even.
Note that the question asks which of the following MUST be true, not which COULD be true.
A. If \(x\) is even, then \(y\) is odd. This is not necessarily true. For instance, take \(x = y = 2\), which are both even.
B. If \(x\) is odd, then \(y\) is a multiple of 4. This is always true because if \(x\) is odd, then for \(xy\) to be divisible by 4, \(y\) must be a multiple of 4.
C. If \(x+y\) is odd, then \(\frac{y}{x}\) is not an integer. This is not necessarily true. For example, take \(x = 1\) and \(y = 4\).
D. If \(x+y\) is even, then \(\frac{x}{y}\) is an integer. This is not necessarily true. Consider \(x = 2\) and \(y = 4\).
E. \(x^y\) is even. This is not necessarily true. For example, consider \(x = 1\) and \(y = 4\).
Answer: B