Solve the Problem $\mathrm { C } ( 6,2 ) = 15$

Solve the problem.
-List all the combinations of 6 objects a, b, c, d, e, and f taken 2 at a time. What is C(6, 2) ?

A) ab, ac, ad, ae, af, bc, bd, be, bf, cd, ce, cf, de, df, ef $\mathrm { C } ( 6,2 ) = 15$
B) ab, ac, ad, ae, af, ba, bc, bd, be, bf, ca, cb, cd, ce, cf, da, db, dc, de, df, ea, eb, ec, ed, ef, fa, fb, fc, fd, fe $C ( 6,2 ) = 30$
C) ab, ac, ad, ae, bc, bd, be, cd, ce, cf, de, df $\mathrm { C } ( 6,2 ) = 12$
D) aa, ab, ac, ad, ae, af, ba, bb, bc, bd, be, bf, ca, cb, cc, cd, ce, cf, da, db, dc, dd, de, df, ea, eb, ec, ed, ee, ef, fa, $\mathrm { fb } , \mathrm { fc } , \mathrm { fd } , \mathrm { fe } , \mathrm { ff }$ $C ( 6,2 ) = 36$