________ scores of times, but Derek still couldn't understand how to use past participle in a co

________ scores of times, but Derek still couldn't understand how to use past participle in a concrete situation．

A.Having explained

B.Having been explained

C.Though it was explained

D.It was explained

答案及解析： D

解析：

因为有连词but，前面应是完整的句子，排除A,B; Though不能和but同时使用，所以选D。