In studying this planet, astronomers have found that life may be possible on Mars. The first..............of this is that Mars has seasons, just like Earth. In other words, Earth's seasons, such as spring or summer, for vegetation and other higher life forms to be found on Mars.
I chose C but the answer is D
Is there any difference between the two words?
I purposely didn't look to see which choice you had made when I read the question.
I chose D immediately but I admit I can't explain why it sounds better to me than C.