The underlined word "undergone" in Line 1 is closest in meaning to ______.

A．the reasons craters are difficult to study
B．the different shapes
C．some features of large craters
D．some difference in the ways small and large craters were formed

A．What astronomers learned from the Surveyor and Apollo space missions.
B．Characteristics of the major terrains of the Moon.
C．The origin of the Moon's craters.
D．Techniques used to catalogue the Moon's land forms.