[求助]黄皮OG一道题，希望大家帮忙看下

其他的就不多说了，直接发题目吧

49 Many house builders offer rent-to-buy programs that enable a family with insufficient savings for a conventional down payment to be able to move into  new housing and to apply part of the rent to a purchase rent.

E programs,which enable a family with insufficient savings for a conventional down payment to be able to moving new housing ,applying

E选项的错误很明显，be able to 和enable意思重复了，可是还有一个地方我不太明白，OG上是这样解释这个选项的：that introduce a restrictive clause that defines programs,which introduce a nonrestrictive clause ,set off by a pair of commas,that may be dropped from the sentence,that is required here because the clause defines programs.

对于这段解释我不是很明白，首先，which 和that引导从句的差别就是限定性和非限定性吗？红色字体的部分没有看明白？能解释一下吗？最后一句话完全没有明白它的逻辑。。。NN帮帮忙啊