1、To our great joy , we have found a big house ______ to store our supplies of food and clothing.
A where B in which 选什么，为什么
2、How about _______ we go to the concert after dinner?
Sounds a good idea.
A if B that 为什么不能用B

第一个答案选A或B都可以.除此之外还可以用we have found a big house to store our supplies of food and clothing in.(把介词放在最后）
第二个选A.因为 how about 是征求对方意见的一种说法。后面为if条件句时，about 后的宾语可以省略。相当于what do you think
of my idea that we will go to the concert after dinner?
这时that 引导的从句为同位语从句，补充说明idea 的具体内容。