题目

Secret codes (密码)keep messages private。Banks, companies, and government agencies use secret codes in doing business, especially when information is sent by computer.    People have used secret codes for thousands of years. _31_ Code breaking never lags(落后) far behind code making. The science of creating and reading coded messages is called cryptography.    There are three main types of cryptography. _32_ For example, the first letters of “My elephant eats too many eels” Spell out the hidden message “Meet me.” _33_ You might represent each letter with a number, F读图，回答下列问题： （1）图中A地的水循环是_________，B地的水循环是_________。 （2）a、b两条河流中，水量较稳定的是_________河，判断依据是_________。 （3）甲、乙两城市中，水资源较丰富的是________城，原因是_________。 （4）水循环得以顺利完成的内因是__________外因是_______________。