og上的例题

第四版og的p131The Great Lake are like this, they were left over from the Plesit--from the Pleistocene glaciers, uh, the Great Lakes used to be a lot bigger as the glaciers were retreating,（1.这里是说原来这里没冰川，，然后冰川retreat的时候，经过这里，是的lakes bigger；还是说，本来冰川就在这，当他离开retreate的时候，lakes会bigger。    2.之所以读不懂，是因为他这里用的搭配：  used to。。。as 。。。was retreating。。。 这个used toi到底是在as的这个进行时之前还是同步？） some of the lakes were as much as a hundred feet higher in elevation.（高度和前面谈的bigger有什么关系？？the bigger, the higher?因为后面一句是(先前e湖高点的时候，他的湖岸在现在湖岸的50公里外)） The beach of a former higher stage of Lake Erie was about fifty miles away from where the beach--the current beach of lake Erie is right now. So I just wanted to tell you a little bit more about glaciers and some positive things uh that we get form climate change, like the ecosystems that develop in these kettle lakesm and how we can look at them in an environmental perspective..


How did the glaciers affect the Great Lakes?
  They made the Great lakes smaller.
  They made the Great Lakes deeper.
  They reduced the biodiversity of the Great Lakes.
  They widened the beaches around the Great Lakes.

解释是“the great lake used to be a lot bigger as the glaciers were retreating”.但是没说选什么。，我不太懂，求指导