From the small number of impact craters on planetary geologists suspect that virtually all its original surface has been covered over by younger volcanic activity

From the small number of impact craters on moon, planetary geologists suspect that virtually all its original surface has been covered over by younger volcanic activity. Most of the Moon's surface is densely covered with craters, but we find relatively few craters within the lunar maria. 

c. magnitude and direction

the components of a vector are the parts of the vectors that are to each other.

b perpendicular