一道高中的英语选择题 大家帮忙解释一下
问题描述：
一道高中的英语选择题 大家帮忙解释一下
This country has high hills ,whose tops spread out into green and moist fields ____ great numbers of sheep are fed.
A to which B upon which C that D which
为什莫？
答
B great numbers of sheep are fed on/upon fields