工业卫生标准规定大气含尘量不允许超过（）毫克/米3。
A、5
B、10
C、20
参考答案：B
用反证法证明命题“一个三角形的三个内角中，至多有一个钝角”的第一步应假设______．
I wanted to have a talk with her, but she ______ me and went away.
A．upset
B．calmed
C．ignored
D．set
