The sale usually takes place outside the house, with the people watching ________ on benches, chairs or boxes.[ ]

A．having seated  

B．seating

C．seated  

D．having been seated

若某处的绝对水位为Mm，该处的当地零点高程为Nm，则该处的当地水位应为（）m。

A、M+N

B、M

C、N

D、M-N