阅读短文，从每题所给的A、B、C和D四个选项中，选出最佳选项。

    Denis was a thief (贼). He wrote a letter to another thief Pat. But the police got the letter. It reads:

   BNLD ZS LHCMHFGS ZMC GZUD SGD SGHQSX ONTMCR VHSG XNT.

   What's it about? How can you read it? The policemen were quick. They studied the letter carefully and soon found out what they want. Now let's suppose (假定) you were a policeman. Can you read Denis' letter?

    Maybe you need some help: When Denis wanted a letter, he wrote down the letter before it in the alphabet (字母表).

    Now can you read Denis' letter? What must Pat do? When? And what did Denis want?