John has $$20,000$ to invest. He invest in Treasury bills that yield $5\%,$ Treasury bounds that yield $7\%,$ and corporate bounds that yield $9\%.$ John wants to have an annual income of $$1280,$ and the amount invested in Treasury bills must be two times the amount invested in corporate bonds. Find the amount in corporate bonds.

a$)$ $$12,000$

b$)$ $$2,000$

c$)$ $$6,000$

d$)$ $$20,000$