At the computer operation test, one is supposed to stay at his own machine, keep his eyes on his screen, ______ to anyone.

A．and not to speak

B．but could not speak

C．instead of speaking

D．or rather speak

下述不属于概念模型应具备的性质是

A．有丰富的语义表达能力

B．易于交流和理解

C．易于变动

D．在计算机中实现的效率高