This is the very plan for the summer holiday ______will be suggested by his cousin.此空格处为何填that 不可填which?谢谢可是为什么呢 请解释的更清楚一些 从语法方面出发

因为有the very ,如果有the first ,the most ,the last,等词，都要用that,换言之，大多数情况that可替换which,反之不行

which前面要有个逗号

因为plan前面有the very修饰,先行词前面有only,any,few,little,no,all,one of,the same,the very等词修饰时 都只能用That
The only thing that we can do is to give you some money.
Li Ming is the only one that got full marks (满分)in our class.
Li Ming is one of the students that want to be teachers in our class.