参考答案

正确答案：B
解析：该题问“根据本段，为什么人们在疲倦时易于发生事故?”短文中说道：When they were tired or careless and they could lead to accidents because they lose track of what we are doing．(当他们疲倦或粗心时，他们就会发生事故。)因此B项“他们忘了正在做的事”为正确答案。