以下那些（）属于系统的物理故障。

A、硬件故障与软件故障

B、计算机病毒

C、人为的失误

D、网络故障和设备环境故障

—I rang you at about nine, but there was no reply.

—Oh, that was probably       I was seeing the doctor.

A．why

B．when

C．what

D．that