(单选题)

What is the fourth paragraph mainly about?

A.The application of the new battery.

B.The characteristics of the paper batteries.

C.The intention to invent the device.

D.The working principle of the invention.

参考答案：A

参考解析：

主旨大意题。根据第四段第一句“This invention is based on an observation made more than a century ago—that microbes（微生物） produce electricity as they digest food.”可知，这项发明是基于一个多世纪以前的观察——微生物在消化食物时产生电能。因此，“Scientists refer to the bio-batteries based on this principle as microbial fuel cells.” 科学家称基于这个原理的生物电池为微生物燃料电池。由此可知，第四段介绍的是微生物燃料电池的工作原理，故选A。

知识点：