6._____ entering the classroom,he found his students all talking and laughing loudly.
问题描述：
6._____ entering the classroom,he found his students all talking and laughing loudly.
a.to
b.for
c.as
d.on
7._____ charcoal and sulphur,potassium nitrate forms an explosive mixture.
a.to combine
b.the combination of
c.when combined with
d.combining
答
6.c
7.d