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? a Surprisingly, vegetation, canals, and traces of intelligent life have not been found.
b In 1971, the largest dust storm ever registered by astronomers covered the whole Martian surface.
c The change in the color of the surface was the greatest Martian mystery.
d According to this theory, the canals were built by Martians to transport water from polar to arid areas.