The first thing my brother is going to do this afternoon is to write a letter.A. whichB.

问题补充：

The first thing my brother is going to do this afternoon is to write a letter.

A. which

B. that

C. why

D. who

答案：

A. which

(答案→)B. that

C. why

D. who

解析：考查定语从句中关系代词。本题为that 引导的定语从句，that修饰thing。