They want to develop a kind of dog-friendly food will help dog owners cool down their pet

问题补充：

They want to develop a kind of dog-friendly food will help dog owners cool down their pets.

A. who

B. what

C. which

D. whether

答案：

A. who

B. what

(答案→)C. which

D. whether

解析：考查定语从句的引导词。句意“他们想研制一种对狗有好处的狗粮，这种狗粮会帮助狗的主人安定他们的宠物。”先行词food指物，排除who；what不能引导定语从句；which指物。故选C。