Read this paragraph from "NASA's Curiosity Rover Finds Clues to How Water Helped Shape Martian Landscape."

"Despite earlier evidence from several Mars missions that pointed to wet environments on ancient Mars, modeling of the ancient climate has yet to identify the conditions that could have produced long periods warm enough for stable water on the surface."

What is the key idea in the paragraph?

A. Earlier Mars missions have suggested wet environments existed in the past.
B. Scientists have yet to prove that lakes existed on Mars.
C. Earlier Mars missions have identified the climate conditions necessary for lakes to exist.
D. Scientists have yet to determine what conditions were necessary for lakes to exist on Mars.