Since two months ago, many waste bottles_____ by the student in Class One. [ ]

A. collected                        

B. have collected

C. were collected                    

D. have been collected

为一患者测得生物剂量是50cm1分钟1MED，现需全身照射，距离改为100cm，照射1/2MED，其照射时间应是()

A．1′

B．1′30″

C．2′

D．2′30″

E．3′