单项选择题双代号时标网络图中表示工作的自由时差的是( )。 A. 实箭线 B. 波形线 C. 斜箭线 D. 虚箭线
单项选择题A person who has been trained in medical science whose job is to treat people who are ill or injured . A. doctor B. medicine C. nurse D. ward