21. Mercury is the smallest and closest planet the Sun. A. from B. to C. toward D. of 22. Theres no water on Mars even though scientists have found traces it. A. on B. with C. for D. of 23. Neptune is so far from Earth, it is _ to the naked eye. A. invisible B. impossible C. disable D. unsuitable 24. Nick saw aliens corning out of the UFO, which a big disc. A. stood for B. consisted of C. looked like D. lived on 25. Could we modify Mars to make it more suitable for human life? A. climate B. trace C. experience D. trek