For positive integers n and k, with k lessthanorequalto n, let F_2 (n, k) denote the set of binary strings of length n, having exactly k ones. For example, F_2 (4, 2) = {0011, 0101, 0110, 1001, 1010, 1100}. a. Give a general expression for the size of F_2 (n, k) in terms of n and k, and compute |F_2 (5, 3)|. b. Let S = F_2(5, 4) Union F_2 (4, 2) Union F_2 (3, 3) Union F_2 (2, 1). Explicitly, S is the following set: {01111, 10111, 11011, 11101, 11110,0011, 0101, 0110, 1001, 1010, 1100, 111, 01, 10} Assuming 0