Hold up 一般指被迫停止而造成耽误,就像例句里车子的行进被堵车被迫耽误(the car was held up by the traffic jam)或比如学生被老师叫住造成学生的耽误（The student was held up by the teacher)等等.
而 Keep back 一般指强行被某人或某事阻挡,如警察阻挡行人或使其退后（The police kept back the passengers)等等.
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