问题
选择题
________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
答案：D
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。