optionaladj.任选的optional charge任选港附加费,选择卸货港费,选择卸货港所增运费optional routine自选节目optional studies选修科目optional service可选择受理的业务optional element可选元素optional command[计] 任选命令optional feature任选功能[部件], 附加设备[部件]optional courses选修课optional counter选择计数器optional culture选择文化