If D is the brother of B
how is B related to C? Which statements are necessary?
1. The son of D is the grandson of C
2. B is the sister of D
I. Only 1
II. Only 2
III. Either 1 or 2
IV. 1 and 2 both are required

Question: 以下哪项是正确的？
Options:
A. 非 III, B. II 或 I, C. 仅 III, D. 仅 I, E. IV 或 II, F. IV 为真
Answer: A
E
F
True statement: IV