A string s is a subsequence of a string t if s is formed from t by deleting 0 or more positions of t. For example, if t = "abcab", then substrings of t include "aba" (delete positions 3 and 5), "be" (delete positions 1, 4, and 5), and the empty string (delete all positions).
a) What are all the other subsequences of "abcab"?
b) What are the subsequences of "aabb"?
c) If a string consists of n distinct characters, how many subsequences does it have?