问题补充:
Scientist are not sure about the of water on the Mars.
A. occupation
B. existence
C. demand
D. treasure
答案:
A. occupation
(答案→)B. existence
C. demand
D. treasure
解析:词义辨析。occupation 职业,占有;existence存在,生存;demand要求,需要;treasure财富,财宝。句意:关于火星上水的存在科学家们不确定。选B。