单选题 Existing low gas mileage vehicles will be ______ phased out over the next five years until transition to more fuel efficient new models is complete.
【正确答案】 B
【答案解析】[解析] 词汇：副词
1．这是一道词汇题，选项均为副词。
2．原文意思是“逐渐淘汰”，因此B．gradually为正确选项。