问题补充：
The reason the people in the mainland go to Hong Kong to buy formula is that people are not convinced the safety of formula produced in the mainland.
A. that; at
B. why; at
C. why; of
D. what; of
答案：
A. that; at
B. why; at
(答案→)C. why; of
D. what; of
解析：句意：大陆的人去香港买婴儿食品的原因是人们不确信在大陆生产的婴儿食品的安全性。第一空是why引导的定语从句，先行词reason，关系词在从句中作状语；第二空是是固定短语be convinced of确信。根据句意故选C。
