Here’s an interesting thought. If glaciers started reforming, they have a great deal more water now to draw on — Hudson Bay, the Great Lakes, the hundreds of thousands of lakes of Canada, none of them / which existed to fuel the last ice sheet — so they would grow very much quicker. And if they did start to advance again, what exactly would we do?
The correct usage in the above sentence is "which" in a grammar book. However, I think both "them" and "which" are correct. Am I wrong?