控制加热炉排烟温度不过低是因为（）
A、控制热效率
B、防止露点腐蚀
C、提高炉负压
参考答案：B
若4cos2α=1，则锐角α=______．
David's brother put ________ in the coffee and drank ________ coffee.[ ]
A. much sugar; much
B. many sugars; a lot of
C. much sugar; a lot
D. many sugars; many
