保压的作用是什么？
参考答案：保压的作用是对白片成形、复制率、脱模有利。
—Where is Zhang Hua? Is he in the classroom now?
—He be there, for I saw him in the teachers’ office just now.
A．can’t
B．may not
C．shouldn’t
D．mustn’t
若a：3=b：4=c：5，且a+b-c=6，则a=______，b=______，c=______．
