A palindrome is a string whose reversal is identical to the string. How many bit strings of length n are palindromes?
Try with N= 2 you get aa,bb 2⌈n⁄2⌉ = 2
Try with N= 3 you get aaa,bbb ,aba , bab 2⌈n⁄2⌉ = 4
Try with N= 4 you get aaaa,bbbb ,abba , baab , 2⌈n⁄2⌉ = 4
A is answer
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