单项选择题
Which of the following statements describe(s) the properties of a purely segmented memory system ？（）
Ⅰ．It divides memory into units of equal size
Ⅱ．It permits implementation of virtual memory
Ⅲ．It suffers from internal fragmentation
A．Ⅰ only
B．Ⅱ only
C．Ⅲ only
D．Ⅰand Ⅲ