optionallyadv.可选择地optionaladj.任选的optional instruction set选择指令组optional sports teaching选项教学optional signed constant任意符号常数optional sampling theorem可选抽样定理optional reject processor可选择剔除处理机optional membership class随意属籍类别optional halt instruction任意停机指令optionalpause instruction暂停指令optional discharging port选择卸货港optional basketball class篮球选项