Scientistsarenotsureaboutthe

Scientists are not sure about the ________ of water on Mars.

A. occupation B. existence C. demand D. treasure

解析：选B。考查名词辨析。句意：科学家们还不能确信火星上存在水。occupation占据，居住；existence 存在；demand要求; treasure珍宝，由句意知B项正确。