The teacher _____ several times, but he still didn't understand it.[ ]

A. explained John the sentence            

B. explained the sentence for John

C. explained to John the sentence          

D. explained about the sentence to John

承包商要求业主提供付款担保，属于承包商的（ ）的风险对策。

A．保险转移

B．非保险转移

C．损失控制

D．风险回避