已知等腰梯形的周长为64，腰长为8，对角线长为28，则连结两腰中点与一底中点的线段组成的三角形的周长为（    ）。

— Our classroom _____ every day.

— So it is. It's our duty to keep it clean and tidy.   [ ]

A. cleans                    

B. is cleaning          

C. is cleaned