Which function models this variation:
a varies directly with b and inversely with c, and when b=10 and c=20, a=14. Using your function, what is the value of a when b=18 and c=9 ?

A. a=b2c;a=1
B. a=b2c;a=2
C. a=2bc;a=1
D. a=2bc;a=2