在关系代数中，对一个关系做投影操作之后，新关系的元组个数( )原来关系的元组个数。

A．等于

B．不等于

C．小于或等于

D．大于或等于

Layer n on one machine carries on a conversation with layer n on another machine. The rules and conventions used in this conversation are collectively known as the layer n

A．topology

B．protocol

C．service

D．peer