The right answer is "The English Channel and North Sea served as barriers to advancing German ground troops."
The Germans had managed to easily surpass the Netherlands, Belgium, and France after a series of lightning strikes, leaving the United Kingdom virtually alone in Western Europe. The German High Command knew of the difficulty and impracticability of an amphibious attack on England while the British Royal Navy controlled the English Channel and the North Sea. On July 16, the German dictator Adolf Hitler ordered the preparation of Operation Sea Lion, the invasion of Great Britain with amphibious and parachute forces. But before an infantry invasion could be authorized, it was crucial that the Luftwaffe conquer air superiority over British skies. In September the RAF, using night bombing, had already thwarted German preparations on the coast, and the Luftwaffe would also fail to incapacitate the British air force. Hitler was forced to postpone and subsequently cancel Operation Sea Lion. Germany suffered heavy casualties, with many qualified pilots and good airplanes being lost, and by the end of October 1940, after three months of operations, they were no longer able to launch daytime airstrikes on the United Kingdom.
