【正确答案】 A

【答案解析】[解析] A)keep up：继续，含有不中断地继续做某事之意。如：
The two friends met again after a long separation．They had a talk over a cup of tea and kept it up till midnight．那两位久别重逢的朋友边饮茶边聊，直至午夜。
B)keep out：把……关在外面，不使入内。如：
My mother shut the door of her study to keep out the nasty cat．我母亲关上了她的书房门，不让那只讨厌的猫进去。
C)keep down：指“抑制(价格、数量)”或“压抑(怒气)”等。如：
Father could not keep down his anger．父亲无法抑制他的怒气。
The government managed to keep the prices down．政府成功地平抑了物价。
D)keep off：离开，不接近。常用于公共场所的牌告等，如：
Keep off the grass! 勿踏草地1