Use the correct form of the past simple passive or the present perfect passive
1. In the First World War, radios ................ (use) to communicate with soldiers
2. Since the 1960s, many radios signals................... (send) into outer space
3. So far, no answers .......................... (receive) from other planets
4. It's possible that the signals have arrived on other planets, but the messages .......................... (not understand)