题目内容：

Police in America have stepped up 1. (they) search for a fifteen-year-old boy called Justin Foster, 2. went missing three days ago in Dover. Witnesses said they saw Justin walking home. Kelly, his sister, said that a bright light woke her up and 3. she once mistook it for the light of the4. (fill) moon. She saw a large spaceship 5. (fly) outside the window. And Justin 6. (shout) then and she hasn’t seen him since. When asked about the 7. (possible) that Justin was taken by 8.(alien), Detective Sam Peterson taking charge of the case said they9.(look) into it and wouldn’t give up 10. they found out the truth.