—Every day, my mother tries to spare some time to chat with me,        she  is very tired after a whole day’s hard work.

—You are lucky to have such a great mum!

A．unless

B．whether

C．although

D．until

以下哪一个不是平台技术实现上的逻辑层次（）

A、持久层

B、业务逻辑层

C、服务交互层

D、实现层