将如下E-R图转换成关系模型，应转换成多少个关系模式？
A．1个
B．2个
C．3个
D．4个
参考答案：C
The letter _______ French. I can't read it. [ ]
A. wrote in
B. is written in
C. wrote on
D. is written on
引风机风门的开度通常根据（）进行调节。
A.给煤量
B.炉膛负压
C.烟气含氧量
D.蒸汽压力
