题干
------I don’t know why he has made such a silly mistake in the exam.
------ He ____________ it before he handed in the paper.
------ He ____________ it before he handed in the paper.
A：may have checked
B：might check
C：must have checked
D：should have checked
答案(点此获取答案解析）
D
(米)和与时间
(秒)的关系为
，则该物体在4秒末的瞬时速度是（ ）
