Three days（ ）usually not enough for a cat to become acquainted w

Three days（ ）usually not enough for a cat to become acquainted with a new cat or kitten.
A、are
B、should be
C、was
D、is
【正确答案】：D
【题目解析】：主谓一致考察：对于一只猫来说，用三天的时间与其他猫或小猫混熟通常是不够的。Three days是一个整体的时间概念，此处遵循“意义一致”原则，谓语动词用单数。