单选题 You don't have to install this radio in your new car. It's an______extra.
【正确答案】 B
【答案解析】解析：A项excessive意为“过多的；过分的”；B项optional意为“选择的；非强制的”；C项additional意为“额外的；附加的”；D项arbitrary意为“霸道的；武断的”。