The continent measures 3,300 miles at its widest point—from St. John's, Newfoundland, to the Pacific Coast. From Alaska to Mexico, the continent extends 4,900 miles. Europe is made up of many small nations. Unlike it, North America is composed of only three large nations: Canada, the United States, and Mexico.

Which opening sentence best states the main idea of the paragraph?

A.

North America stretches "from sea to shining sea."

B.

Mexico is well-known for having dry, hot summers.

C.

Those countries have many people and different climates.

D.

Canada is the largest country in North America.