Police in America have stepped up their search for a fifteen-year-old boy who went missing two days ago. Every one has shown great interest in his disappearance due to sightings of puzzling lights in the sky. Some people in Dover also say that they have seen aliens. Police have not ruled out the possibility that Justin was taken by aliens. Photos have been taken and samples have been collected from the plants, but no evidence of life has been discovered. Who else has been taken by the aliens.

Present perfect tense (现在完成时)

Present perfect tense: have/has + 过去分词 被动语态： have/has + been + 过去分词