若集合A1,A2,A3是集合B的一个划分,则A1,A2和A3三个集合的交集是
A1
B. A2
C. ¢
D. A3
若关系R是集合A和集合B的笛卡尔积,则关系R中的元素是有序对偶
A. 对
B. 错
The Double Seventh Festival, on the seventh day of the seventh lunar month, is a traditional festival full of ________.
The seventh day of the seventh lunar month has been regarded as _______.