Answer: Washington, D.C.
Explanation:
The capital city of the United States is Washington D.C. so if the capital city of the U.S. was also the capital city of Canada, Canada's capital would be the U.S. capital of Washington D.C.
This might have been the case if the U.S. had managed to incorporate Canada into the United States during the American Revolution and the subsequent, War of 1812.
