问题补充：
The dog is very smart, but sometimes it’s boring.
A. a kind of
B. kind of
C. kinds of
D. all kinds of
答案：
A. a kind of
(答案→)B. kind of
C. kinds of
D. all kinds of
解析：A. a kind of 一种 B. kind of 有一点 C. kinds of 各种各样的 D. all kinds of各种各样的；句意：这条狗非常聪明，但是它有时候也有点让人厌烦。故选B
