Suppose Alice has a block cipher with 250 keys, Bob has one with 240 keys, and Carla has one with 230 keys. The only known way to break single encryption with each system is by brute force, namely trying all keys. Alice uses her system with single encryption. But Bob uses his with double encryption, and Carla uses hers with triple encryption. Who has the most secure system? Who has the weakest? (Assume that double and triple encryption do not reduce to using single or double encryption, respectively. Also, assume that some plaintext-ciphertext pairs are available for Alices single encryption, Bobs double encryption, and Carlas triple encryption.)