【单选题】First Planet from the Sun Mercury, the planet nearest the sun, is difficult to observe from the Earth because it rises and sets within two hours of the sun. ______【46】 Planetary scientists can estimat...
A.
Some regions on Mercury are heavily cratered, suggesting that they are very old surfaces that were probably formed about 4 billion years ago
B.
The crater was probably created by the impact of a large planetesimal when Mercury was forming
C.
Consequently, little was known about the planet until the Mariner 10 spacecraft made several flybys in 1974 and 1975
D.
Mercury is much smaller than the Earth
E.
The surface gravity is about one-third as strong as Earth's, and a thin atmosphere surrounds the planet
F.
These plains have relatively few impact craters (46)