Knowing their son failed in the exam, the parents stood there, ________.

A．looked disappointed

B．looking disappointed

C．looked disappointing

D．looking disappointing

电子皮带秤日常与定期维护要求：皮带秤的实煤检测应每月不少于（）次，使其动态测量精度保持在最大允许误差范围以内；每隔（）年应安排一次大修，对现场安装部件进行检查、加油或更换；消除原安装质量不符合JJG650-90规定或影响皮带秤长期稳定性的不良因素；每年及检修后，应按国家计量检定规程的要求进行周期检定。

A.2；1－3

B.3；1

C.1；2

D.4；4