If the sum of `n` terms of an A.P. is given by `S_n=a+b n+c n^2, w h e r ea ,b ,c` are independent of `n ,t h e n` (a) `a=0` (b) common difference of A.P. must be `2b` (c) common difference of A.P. must be `2c` (d) first term of A.P. is `b+c`