Which of the Following Is NOT One of the Key

Multiple Choice

Which of the following is NOT one of the key reasons that so many spacecraft from Earth have visited Venus and Mars?

A) they are the two planets closest to the Earth
B) they resemble the Earth in a number of significant ways
C) their atmospheres are very similar to the Earth in terms of their chemical make-up
D) both have tantalized us when seen through a telescope -- one because it is perpetually covered by clouds, the other because it shows seasonal changes
E) they each have a solid surface on which we can trace their geological history