______ is permanently stored in the computer and provides a link between the hardware and other programs that run on the PC.

A．Interface

B．Operating system

C．Internet

D．Applicaion software

哪一项不是肝血管瘤的声像图表现 （

A．边界清、锐的强回声结节

B．绕有强边的网格状回声

C．有相通或环绕的血管

D．回声均匀的团块伴后方回声衰减

E．回声团块具有可压缩性