But I always knew when Doug was playing the tympani behind me because it was flawless . I have a memory of Doug playing the rudiments on the snare drum for a music class test . The younger drummers could only wonder at his humble performance as I watched from across the room , mesmerized . At heart , Doug was much more of a set player , a rocker , but his formal technique was amazing .

Question: How can the narrator be so sure that Doug is behind him ?
Answer: None of the above choices .The narrator has heard Doug 's virtuoso playing before .The narrator can hear Doug 's familiar voice .The narrator can smell Doug 's familiar odor .