It may be necesssary to stop ___in the learing process and go back to the diffeult points in the

lessons
a.at a distance b.at intervals c.at case d.at length
选A。什么意思？每个选项怎么解释

It may be necesssary to stop at a distance at a distance in the learing process and go back to the diffeult points in the lessons
可能有必要学一定的内容后再停下来，回顾（温习）一下本课的难点.

a.at a distance :在相当的距离，这里指学到了一定的量.
b.at intervals，时不时，经常
c.at case ，在...情况
d.at length，最后，终于

同问。。。