20. (Part-1): If a Deterministic Turing Machine M is programmed to check if two strings w1 and w2 (both of length N) are equal by zig-zagging along the string, the machine M will take this much runtime in Big-O notation. (Part-2): A TM that writes into O(N3) cells at least once must take this much time. (a) O(N) for Part-1. (b) O(N2) for Part-1. (c) At least O(N3) time for Part-2. (d) Can be lower than O(N3) time for Part-2.