【正确答案】 D

【答案解析】解析：command n.掌握，运用能力；命令，指挥(如：He has[a]good command of spoken French．Who issued the command to fire?take／assume／be in command of；He has a hundred men under his command．在他的指挥下。I'm at your command．听候你的差遣。It was done by the Queen’s command．根据女王的命令。)proficiency n．熟练程度，掌握程度，精通。efficiency n．效率；功效。effectiveness n.效力，有效。