Rules: (1) If A joins, B does not. (2) If C joins, then D joins. (3) Exactly three members join from {A,B,C,D,E}. If A and C both join, who else must join?

✓ Correct Answer: Option B

A joins → B does not (Rule 1). C joins → D joins (Rule 2). So A, C, D join (3 members). But we need exactly 3, and that's A, C, D = 3. Wait, plus E? A+C = need third. C→D, so D joins. A,C,D = exactly 3. But option says D and E. Hmm, A joins, C joins, D joins (from rule 2) = 3 exactly. No room for E. So answer should be D only? But "who else must join" besides A and C — D must join. That's D and E: no, just D. Answer is D only.