They are at the doctor's中的at the doctor's如果是就医的意思,at是动词吗?为什么不用加ing?

问题描述:

They are at the doctor's中的at the doctor's如果是就医的意思,at是动词吗?为什么不用加ing?
再对at the doctor's提问.

at the doctor's "在医生的诊所",at不是动词,是介词.
Where are they?