B, according to the title map, ② In125th generation, the gene frequency of A is 75%, that of A is 25%, and that of aa is 25% × 25% = 6.25%. So b is wrongly described.

After C, 150 generations, the gene frequencies of the three populations are quite different, and reproductive isolation may occur, so C is described correctly.

D. According to the title map, with the increase of population algebra, the frequency of 3A gene is almost unchanged, so the description of D is wrong.

So, C.