It was from John, gave me a big key to my dad.
A. who; understand
B. that; understanding
C. who; understanding
D. which; to understand
网友回答
A. who; understand
B. that; understanding
(答案→)C. who; understanding
D. which; to understand
解析：考查定语从句引导词的区别。它就是来自于给予我明白我父亲一个关键点的约翰。先行词John是人，引导定语从句的关系代词可以用that或who，但约翰后有逗号隔开，后面的从句是非限制性定语从句，故不能用that引导非限，只能用who；a big key to ……“……的一个关键”中的to是介词，to后面要用动词ing形式；故C 正确。