2.there's no water on mars even though scientists have found traces..............it. A.on B.with C.for D.of 3.neptune is so far from earth, it is.............to the naked eye. A.invisible B.impossible C.disable D.unsuitable 4.nick saw aliens croning out of the UFO, which...........a big disc. A.stood B.consisted of C.looked like D.lived on 5. could we modify mars'................to make it more suitable for human life ? A.climate B.trace C.experience D.trek 6.when do you think the first human..............walk on the surface of mars? A.must B.should C.would D.might