血压通常是以右上臂肱动脉血压为标准。正常成人安静是舒张压为12.0~18.6千帕，收缩压为8~12千帕，脉压差为4.0~5.3千帕。

Stone seldom, _______, refuses to help others when they turn to him, for he is only too ready to do so.

A．if never

B．if not

C．if ever

D．if any