填空题
What should you do to protect your computer from a virus You must make sure that you have an （）.
antivirus program
单项选择题 Some scientists think there may have been life on the Mars in the past because （）.
单项选择题 I am very grateful to you for what you"ve given me and ______ you have done for me.
单项选择题 Jean did not have time to go to the concert last night because she was busy ______ for her examination.
