1. Make two auxiliary lines first: cross O to make BC's vertical line, cross BC and m; Extend AO, through BC and n.

2.∠∠a and ∠B are both 60, ∴AB=AN=BN= 12.

3.∫∠ANB = 60，ON= 12-8=4 .∴MN=2,BM= 12-2= 10

4.∵BM=CM (vertical diameter theorem) ∴BC=20∴ Choose D.

I made it myself. I think so.