把两个及其两个年级以上的儿童编在一个班级，直接教学与布置、完成作业轮流交替进行，在一节课内由一位教师对不同年级学生进行教学的组织形式是复式教学。( )

Wind power is an ancient source of energy        we may return in the near future.

A．on which

B．by which

C．to which

D．from which