Suppose X distributed as normal ( ia, 1) , i= 1. .... n are independent random variables. The maximum likelihood estimator of a is O a. n 6 E (ix,) n (n + 1) (2n + 1) i=1 Ob. 6 E (ix,) (n + 1) (2n + 1 ) O c. 6 E (ix.) n (2n + 1) i=1 O d. 6 E (ix.) n (n + 1) 1=1