题干
In communication, a smile is usually ________ strong sign of a friendly and ________ open attitude.
A：the, /
B：a, an
C：a, /
D：the, an
答案(点此获取答案解析）
C
In communication, a smile is usually ________ strong sign of a friendly and ________ open attitude.
A：the, /
B：a, an
C：a, /
D：the, an
