【单选题】

The Moon, which has undergone a distinct and complex geological history, presents a 　　　 striking appearance. The moon may be divided into two major terrains: the maria (dark 　　　 lowlands) and the terrace( bright highlands). The contrast in the reflectivity (the capability 　　　 of reflecting light) of these two terrains suggested to many early observers that the two (5)　 terrains might have different compositions, and this supposition was confirmed by missions to 　　　 the Moon such as Surveyor andApollo. One of the most obvious differences 　　　 between the terrains is the smoothness of the maria in contrast to the roughness of the 　　　 highlands. This roughness is mostly caused by the abundance of craters; the highlands are 　　　 completely covered by large craters(greater than 40-50 km in diameter), while the craters (10)　 of the maria tend to be much smaller. It is now known that the vast majority of the Moon’’s 　　　 craters were formed by the impact of solid bodies with the lunar surface.　　　　　 Most of the near side of the Moon was thoroughly mapped and studied from telescopic 　　　 pictures years before the age of space exploration.Earth-based telescopes can resolve 　　　 objects as small as a few hundred meters on the lunar surface.Close observation of (15)　 craters, combined with the way the Moon diffusely reflects sunlight, led to the 　　　 understanding that the Moon is covered by a surface layer, or regolith, that overlies the 　　　 solid rock of the Moon. Telescopic images permitted the cataloging of a bewildering array 　　　 of land forms.Craters were studied for clues to their origin; the large circular maria were (20)　seen. Strange, sinuous features were observed in the mari

A、Although various land forms 　　　 were catalogued, the majority of astronomers’’ attention was fixed on craters and their 　　　 origins.　　　　　Astronomers have known for a fairly long time that the shape of craters changes as 　　　 they increase in size. Small craters with diameters of less than 10-15 km have relatively (25)　simple shapes. They have rim crests that are elevated above the surrounding terrain, 　　　 smooth, bowl-shaped interiors, and depths that are about one-fifth to one-sixth their diameters.　　　 The complexity of shape increases for larger craters. According to the passage, the maria differ from the terrace mainly in terms of
A、age
B．manner of creation
C．size
D．composition