阅读理解。 David's got a text message from his friend Sam. Can you understand it? 7,15      20,15      20,8,5      16,1,18,11 This message was in code (密码, 代码). How many letters are there in the English alphabet? Twenty-six. So can you read it now? A B C D E F G H I J K L M N O P Q R S T U ... 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 ... David decided to use a different code in his answer to Sam. This time, he was putting A instead of B, B instead of C, and so on. His message to Sam shown on the right was very short. Can you understand? A B C D E F G H I J K L M N O P Q R S T U V W X Y Z B C D E F G H I J K L M N O P Q R S T U V W X Y Z A Sam guessed David's code, and he replied: ZESDQ, RBGNNK. When David's little sister saw the message, she felt puzzled. She couldn't read it at all. But David was very happy that his sister couldn't read his message! Lots of people use codes to send secret messages. In a war, secrets are very important so soldiers often send each other messages in code. Nowadays, we can use computers to make and break difficult codes. Even before computers, people invented other machines to do this. Here's a famous machine that Germans used in the Second World War (1941-1945). It's called the Enigma Machine. The machines were very big, heavy and expensive. Every day they changed the codes automatically, and this made it almost impossible for anyone to break the codes. The person sending a message had one machine, and the person receiving the message had another. 1．Sam asked David to go to the ______. [     ] A．park B．house C．museum D．zoo 2．Sam told David to meet him ______. [     ] A．before breakfast B．after school C．after dinner D．before school 3．Codes are used to keep something ______. [     ] A．new B．old C．secret D．known 4．People will never know the meaning if they can't ______ the code. [     ] A．break B．make C．change D．write 5．The best title of this passage is . [     ] A．English Letters B．Codes C．Messages D．Machines