This should be a problem that can be solved by the frequency distribution map of grade three.

Frequency of each group = frequency/total number of people; The sum of the frequencies of each group is 1,

Let the frequencies of the first group, the second group and the third group be k, 2k and 3k respectively.

Then k+2k+3k+5× 0.0375+5× 0.0125 =1,that is, 6k+0.25= 1.

The solution is k=0. 125.

So the frequency of the second group is 2k=0.25.

Number of people selected = 12/0.25=48.

So choose B.