1 . Not very long ago, the only way to stay in touch with faraway friends was writing letters that took ages to be_________. (根据句意填空)

2 . This is a new topic and it needs to be ________ (探究) in depth. (根据汉语提示单词拼写)

3 . The man hurried to the airport, only________________(tell) the plane had left. (所给词的适当形式填空)

4 . He hurried to the station, only ________ ( tell ) the train had left. （所给词的适当形式填空）

5 . What if the only way of getting news from faraway friends was writing letters that took ages to be _________ (递送)?（根据汉语提示单词拼写）