If x is a string, then xR is the reverse of the string. For example, if x = 1011, then xR = 1101. A string is a palindrome if the string is the same backwards and forwards (i.e., if x = xR). Let B = {0, 1}. The set Bn is the set of all n-bit strings. Let Pn be the set of all strings in Bn that are palindromes.

(c) Determine the cardinality of P7 by showing a bijection between P7 and Bn for some n.