The boss believed Tom's reason for being late for work is he had to help his grandma cook

问题补充：

The boss believed Toms reason for being late for work is he had to help his grandma cook.

A. why

B. that

C. where

D. because he

答案：

A. why

(答案→)B. that

C. where

D. because he

解析：考查表语从句。That he had to help his grandma cook是一个表语从句，这个句中结构很完整，that只起引导作用，不充当任何成分。