It is the firm belief of astronomers【C1】______there are living creatures on other planets.

It is also their ear- nest wish that some day【C2】______may be made with such living creatures【C3】______sending messages in the form. of radio signals. The【C4】______of communicating with people on other planets may one day come【C5】______since radio telescopes【C6】______.

Scientific project of various kinds are now being launched to【C7】______signals or to receive signals from distant planets. The question now【C8】______: What sort of message【C9】______so that it could be understood？ To send a message in any language would be impractical【C10】______it would【C11】______not be intelligible.

It is the opinion of scientists that a signal in the【C12】______of a simple arithmetic sequence might be understood.

Scientists think that pictures might also be understood, so it【C13】______a good idea to send pictures of the people【C14】______our planet. Pictures of domestic animals together with【C15】______crops we【C16】______for our food might also be【C17】______. Pictures of houses and buildings might【C18】______further information about our life and society.

As time goes on, TV pictures might be sent, which would【C19】______acquaint the beings on other planets【C20】______life and the level of civilization on our planet.

【C1】

A．which

B．what

C．why

D．that