Thanks to scientific research we now have a good understanding of an earthquake is and w

问题补充：

Thanks to scientific research, we now have a good understanding of an earthquake is and why it happens.

A. how

B. which

C. whether

D. what

答案：

A. how

B. which

C. whether

(答案→)D. what

解析：经分析，of是介词，后接宾语从句，而is是系动词，后接表语，故应选D。本句含义，多亏了科学研究，现在我们对什么是地震以及为什么发生地震有了很好的了解。