Directions：
You will hear three dialogues or monologues．Before listening to each one，you will have 5 seconds to read each of the questions which accompany it．While listening，answer each question by choosing A，B，C or D．After listening，you will have 10 seconds to check your answer to each question．You will hear each piece bonce only．
Questions11-23 are based on the following introduction to balloons．You now have 15 seconds to read Questions 11-13． The common use of unpiloted balloons is______．