收益减去成本等于：( )

单项选择题Bert says that a charged particle in a vacuum can travel in a helix only if a uniform electric field and a uniform magnetic field are both present and both parallel to the axis of the helix. Stuart says that only a magnetic field with a component parallel to the axis of the helix is needed. Which one, if either, is correct, and why?

单项选择题承揽人经定作人同意将接受的任务转让给第三人后，应由( )完成的工作成果向定作人负责。