According to the least common multiple, we can get1* 90 = 90,2 * 45 = 90,3 * 30 = 90,5 *18 = 90,6 *15 = 90,9 *10.

In these formulas, 1, 2, 3, 4, 5, 9, 10,15,45 is not a multiple of 6, and the remaining 6,18,30,90 is a multiple of 6.

So there are four pairs, which are 6 and 18, 6 and 30, 6 and 90, 18 and 30 respectively, and the least common multiples of 6 and 18, 6 and 30 are 18 and 30, so it is excluded.

Choose C.