An operating system is (74) .

A．the highest layer of software

B．the lowest layer of software

C．the highest layer of hardware

D．the lowest layer of hardware

居住区内道路可分为几级

A．主干路、次干路和支路三级

B．干路，支路和小区路三级

C．居住区道路、支路、小区路和组团路四级

D．居住区道路、小区路、组团路和宅间小路四级