This is easy.
Ask this one question: "Do you believe the WTS is inspired and/or directed by Jehovah?"
If the answer is yes, the follow-up question is, "Then why have the WTS prophecies down through the years (1914, 1925, 1975, etc.) been so consistently wrong?"
If the answer is no, the follow-up question is, "So why would you belong to an organization you admit is not inspired and/or directed by Jehovah, and in addition, has been consistently wrong with their prophecies down through the years?"