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

The first thing my brother is going to do this afternoon is to write a letter.
A. which
B. that
C. why
D. who

A. which(答案→)B. thatC. whyD. who解析：句意：我的弟弟今天下午打算做的第一件事情就是写一封信。定语从句用来充当句中定语的成分，被修饰的名词叫先行词，连接主从句的是关系词。关系词分：1关系代词：who , whom , whose , which , that 。2关系副词：where,when why 等。一般的who 用于指代人的先行词，which 用来指代物的先行词，that既可指认又可指物。关系代词在从句中作主语、宾语。但在下列情况下一般只用that而不用which ：先行词本身是all , everything , something , nothing , anything等不定代词时， 先行词已有序数词或形容词的最高级或the last, the only等作定语时，先行词既有人又有物时。故选B。

和我的回答一样，看来我也对了