仓储机械一般具有（）等特点。

A.安全性、节能性、环保性和经济性要求高

B.机械化、自动化程度高

C.专业化、标准化程度低

D.作业场所不固定

Without the keys, the manager waited in front of the office ________ was locked ten minutes ago.[ ]

A. who

B. which

C. /

D. whom