选what.
这个句子含有两个从句.首先是what引导的宾语从句.即they reached what they thought was the place.其中,引导词what 在从句中又充当主语成分.即实际上是what was the place…….they thought是在关系词与从句句子成分之间插入的“人称代词+think/believe/guess等表示猜测的态度的句子.这是混杂式的从句.其次,place 后面又有一个定语从句.定语从句的引导词that因在从句中充当宾语,即充当dream of 的宾语,所以that 省略.