Complete the paragraph. Use the correct form of the past simple passive or the present perfect passive. In the First World War, radios ...(use) to communicate with soldiers. Since 1960s, many radio signals ...(send) into outer ...

Complete the paragraph. Use the correct form of the past simple passive or the present perfect passive. In the First World War, radios ...(use) to communicate with soldiers. Since 1960s, many radio signals ...(send) into outer space. So far, no answers ...(receive) from other planets. The signals have arrived on other planets, but the massages ...(not understand).