The main idea of "The Difficult Road to Mars" is that people have been looking for evidence of life on Mars--and finding it--throughout history. Which sentence provides the strongest textual evidence for the article's main idea? In 1971, the largest dust storm ever registered by astronomers covered the whole Martian surface.

The change in the color of the surface was the greatest Martian mystery.

Surprisingly, vegetation, canals, and traces of intelligent life have not been found.

According to this theory, the canals were built by Martians to transport water from polar to arid areas.