题目

If you and your friends wish to share a secret, you can write it in code, and no one else will be able to read it. Codes are one way of writing in secret Ciphers(暗码)are another. In a code each word is written as a secret code word or a code number. In a cipher each letter is changed. Codes and ciphers have played an important role in the history of the world. Julius Caesar,the Roman ruler who defeated almost all the countries in Europe about 2,000 years ago, used a cipher when he sent secret messages to his troops. During the American Revolution, George Washington’s spies used a ________ the tower building, where you see the whole city． [　　] A．Standing on the top of B．If you climb to C．When you reach the top of D．Get to the top of