___mathematical concepts of some students was very difficult because the students did not have a math background .a Explain b explaining c have explained d having explained

___mathematical concepts of some students was very difficult because the students did not have a math background .
a Explain b explaining c have explained d having explained

因为做主语
所以A和C淘汰了
要是动名词才能作主语
我觉得选B吧
向学生解释数学的内容是很难得因为学生没有学习数学的背景.