在过渡过程中，由高频振荡电流过零时刻而熄灭的电弧，叫（）。
A.工频电弧
B.高频电弧
C.开放电弧
D.油中电弧
参考答案：D
1956年，里格斯通过把社会划分为______而第一次使用了他那广为人知的模型。
On some projects, especially ones of smaller scope, activity sequencing, activity resource estimating, activity duration estimating, and（）are so tightly linked that they are viewed as a single process that can be performed by a person over a relatively short period of time.
A．time estimating
B．cost estimating
C．project planning
D．schedule development
