主句没有宾语啊,宾语一般应该是个名词或名词性物主代词。所以引导从句的应该是一个可以代表名词的特殊疑问词,就是what。
而which不引导名词性从句,引导定语从句。就是当主句中已经有宾语的时候才会用到which。对于本题就是改成“A modern city has been set up in a place ____ was a waterland ten years ago."这时才会填which。which引导定语从句修饰时什么样的place。而原句中连place都没有,就根本谈不到是什么样的place了,所以也就用不上which。
有个类似的句子,你应该碰到过:
The village now is no longer what used to be.
如果用which的话必须有先行词,如
A modern city has been set up in a place which was a wasteland.
这种句式一般用what简洁