Ever since man began to use the telephone, there have been new problems arising from the carrying of messages. At first, each message was carried by a pair of overhead wires. As a result, telephone exchanges were soon surrounded by thousands of wires. The wires were then replaced by cables  ,each containing many pairs of wires. Each cable is capable of carrying many messages. These cables, laid underground, replaced the overhead wires. The more _ telephone services have become, the more demand for these services has increased, particularly the demand for long-distance services. In China, for example, this growth is now over 30% every year. Long-distance telephone exchanges are usually in crowded cities, where is not easy to lay new labels for expanding services. The use of radio to send telephone message and to link all telephone exchanges makes it possible to get rid of overhead wires and some underground cables. Now man has invented the microwave system (  ). In a microwave system messages from various places can be brought together, and then they are sent out and received by radio. After that, the messages are split into their original form. Finally, they are sent to the places where they are going to be sent. What does the word  _  mean?
growing using holding understanding