example of John shows that knowledge of first aid can help a lot.A. The; a B. An; 不填C. A

问题补充：

example of John shows that knowledge of first aid can help a lot.

A. The; a

B. An; 不填

C. An; a

D. The; 不填

答案：

(答案→)A. The; a

B. An; 不填

C. An; a

D. The; 不填

解析：考查冠词的用法;第一空填the，特指约翰的例子，第二空填a，a knowledge of first aid指急救知识，句意是：约翰的例子表明急救知识帮助很大。选A。