Suppose  $$A$$  and  $$B$$  are the coefficients of $$30^{\text{th}}$$  and $$12^{\text{th}}$$  terms respectively in the binomial expansion of  $$(1+x)^{2n-1}.$$ If $$2A=5B,$$  then  $$n$$  is equal to: 

A

22

B

20

C

21

D

19