语法与词汇 9.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】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”。