题目内容：
Bridging the GapDirections: Fill in the blanks with a word or a phrase that best completes the passage. You may choose a suitable one from the list
low compared to personal another require
running out of refers to economic positive limited
Often referred to as green transport, sustainable transportation is the way of the future. Transport powered by gas and petrol is running on a__①__ time frame, as both these sources are unsustainable in the long run. In contrast, sustainable transportation offers an option to a world rapidly__②__ renewable fuel sources.
参考答案：
答案解析：