未知题型 We may learn from the passage that ______.A．people do not care about the removal of barriers between countriesB．people can not remove the obstacles made by themselvesC．man-made barriers are sometimes harder to overcome than natural onesD．barriers should be taken for granted as they always exist

单项选择题 “勤”所指的表现是指()

未知题型 常用的平均指标有( )。