最小二乘法是根据历史数据拟合出一条发展趋势线，使该线与实际值之间的（）为最小。
A、离差之和
B、离差平方和
C、方差之和
D、方差平方和
参考答案：B
下列关于实验室制取氧气的说法中．一定正确的是 [ ]
A．必须选择含氧元素的物质作原料
B．必须用排水法收集氧气
C．必须使用二氧化锰作催化剂
D．必须使用酒精灯
It is required by the authorities of school that students should be treated ______ by their teachers but how can a teacher respect them when the students show no respect for him? [ ]
A. in respect
B. with respect
C. for respect
D. by respect
