题中用was reading,这个while在题中引导的状语从句强调“某个动作正在进行的同时,另一个动作也在进行”.while这种情况下通常在句首.“While two detectives were keeping guard at the door,two others opened the parcel.”这句中的while引导的是时间状语从句,表示“当...的时候”,“与...同时”,可放句首或句中,可用when代替.