–I argued with my best friend. What should I do?– you feel sad you should write a letter

问题补充：

–I argued with my best friend. What should I do?

– you feel sad, you should write a letter and say sorry.

A. But

B. If

C. So

D. Unless

答案：

A. But

(答案→)B. If

C. So

D. Unless

解析：But但是，可是；If是否，如果；So因此；Unless除非，如果不。句意：我和我最好的朋友吵架了。我应该怎么做？——如果你感到难过，你应该写一封信说抱歉。结合语境可知选B。