—Chinese people are worried about the food they eat these days.

—Yes, the food safety is a serious         in our country. We should try to solve it.

A．subject

B．program

C．problem

D．opinion

从所给的四个选项中，选择最合适的一个填入问号处，使之呈现一定的规律性。（）

A．A

B．B

C．C

D．D