Question about melting of laurentide ice sheet?

Question

As the Laurentide Ice Sheet began to melt from its furthest point south in OH, PA, and NY, why did the melt water first flow to the Mississippi River and the Hudson River since the water does not do that now?
a.) The Great Lakes basins and the St. Lawrence River had not yet formed
b.) the Great Lakes basins and the St. Lawrence River were still filled with ice
c.) moraines and glacial debris blocked the water from flowing to the St. Lawrence when the glaciers first started to melt
d.) Sea level was 300 feet lower and this controlled the flow of the melt water

Answer 1

C is correct, once the sheet melted back enough the water poured out into the St. Lawrence. check out the journal Paleooceanography

Answer 2

THAT MUST BE PART OF GLOBAL WARMING