题干
—Jack, I have________ working out the math problem.
—Don't worry. Let me help you.
A：fun
B：trouble
C：experience
D：interest
答案(点此获取答案解析）
B
—Jack, I have________ working out the math problem.
—Don't worry. Let me help you.
A：fun
B：trouble
C：experience
D：interest
（R或R´可以是烃基或H原子）。