His ________behavior made everyone nervous. He was always rushing to open doors and perform other small tasks, apologizing unnecessarily for any inconvenience that he might have caused.

A．oblivious

B．observant

C．obsequious

D．obsolescent

循环式制动压力调节器是在制动总缸与轮缸之间（）一个电磁阀、直接控制制动轮岗的压力。