15.“Most dog owners are convinced that their four-legged friends know exactly what they mean when they use certain words like sit， stay or treat.”The function of the underlined part is .（ ）

A.a subject clause

B.an adverbial clause

C.an object clause

D.an attributive clause

15.B【解析】句意：大多数狗的主人都相信当他们使用某些词，如坐、住或招待时，他们的四脚朋友知道他们的意思。由句意可知，画线部分为when引导的时间状语从句。故选B。

16.— Peter graduated from college at a very young age.

— He _____ have been a clever student.（ ）

A.could

B.must

C.should

D.might

16.B【解析】could/might “可能”；should“应该”；must“一定”；题干上句句意为“Peter年纪很小就从大学毕业了”，因此下句应接表示肯定的推断“他一定是个聪明的学生”。故选B。