问题补充：
Sam felt he had nothing to the discussion, so he left early.
A. belong to
B. lead to
C. adapt to
D. contribute to
答案：
A. belong to
B. lead to
C. adapt to
(答案→)D. contribute to
解析：句意，Sam感觉他无法做任何事有助于谈论，所以他早早就离开了。belong to属于 lead to导致 adapt to使适应于 contribute to有助于，根据题意，故选D。
