Three days ______ usually not enough for a cat to become acquai

Three days ______ usually not enough for a cat to become acquainted with a new cat and or kitten.
A、 are
B、 should be
C、 was
D、 is
【正确答案】：D
【题目解析】：该题考查的是主谓一致性和时态，时间距离等单词做主语时，通常情况下用单数，因为陈述的为一般性事实，所以用一般现在时态，选项D为正确答案。