复合式气体检测仪充满电后可（）小时连续操作。
A、10
B、20
C、30
D、40
参考答案：B
直线过点 (－3,－2)且在两坐标轴上的截距相等，则这直线方程为 .
Generally speaking, when the Herfindahl-Hirschman Index (HHI) is calculated by summing the squared percentage market shares, how many largest firms data is used in a industry, supposing there is a large number of firms in the industry:（）
A．4
B．10
C．50
