在国际中长跑比赛中，一些来自东非高原的选手常取得好成绩的原因是[ ]

A、他们长期适应东非高原热带草原地理环境的结果

B、他们为了改变贫困的生活，努力训练的结果

C、他们以野生动物为主要食物的结果

D、东非高原海拔高，空气稀薄，所以，东非高原的选手耐力好。

As an operating system repeatedly allocates and frees storage space, many physically separated unused areas appear. This phenomenon is called __( )__.

A．fragmentation

B．compaction

C．swapping

D．paging