two 's complement

Question

Using binary arithmetic, a number c is computed by taking the n-bit two’s complement of ‘a – b’. If n is eleven, a = 10100001001 and b = 10101 then c = ?
(A) 011000011112
(B) 011000011002
(C) 011000111002
(D) 010001111002

explain?

Comments

Using "Binary arthimatic" which is applied on binary no. How "2" (in the end of the options) came into the picture? If you remove "2" I will go for Option B.

Answer 1

 10100001001

-00000010101

______________

(10011110100)   

1's is 01100001011

2's is 01100001011

                            +1

________________

         01100001100

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