阅读下面短文并回答问题，然后将答案写到答题卡相应的位置上（请注意问题后的字数要求）。
〔1〕Secondhand smoke is a byproduct of cigarette, cigar or pipe smoking. Secondhand smoke occurs when tobacco burns or when smokers exhale, and it is inhaled involuntarily by non-smokers.
〔2〕Secondhand smoke___________________. The first is called side stream smoke, which is the smoke released from the burning end of a cigarette or cigar, or from tobacco burning in the bowl of a pipe. The second is called mainstream smoke, which is exhaled by a smoker.
