__________是我国各级各类学校最常用的、基本的教学组织形式。 ( )
A.个别教学
B.班级授课制
C.分组教学
D.复式教学
参考答案:B
A. waited B. was waiting C. had been waiting D. had waited
请你说明什么是清洁提单和不清洁提单