根焊
参考答案：为管与管、管与管件或管件与管件之间焊接时的第一层焊缝。
如图所示，真空中O点处固定一点电荷Q，同时在O点通过绝缘细线悬挂一带电荷量为q质量为m的小球，开始时细线与小球处在水平位置且静止，释放后小球摆到最低点时，细线的拉力为4mg，则固定电荷Q在最低点B处产生的场强大小为（ ）
A．mgq
B．2mgq
C．3mgq
D．2mgq
In the short run, if price is below average total cost (ATC), the firm will:（）
A. keep running as long as it is covering its variable costs.
B. produce more.
C. raise prices.
