There are four things that scientists think life needs. One of the most important things is (51)________ water (H2O). This can be on the surface or under the surface. The temperature of the planet or moon should be (16)________ to keep water in liquid form. Gases are also needed for life, (17)________ carbon (C), hydrogen (H), Oxygen (O), and nitrogen (N). (18)________ should also be an atmosphere which is the name for the nutrients around the planet or moon. A thick atmosphere helps to keep the planet or a moon warm, (19)________ stop space rocks and other dangerous things falling (20)________ the space. A thin atmosphere does not. 15. A. liquid B. gases C. solid D. mineral 16. A. enough warm B. warm enough C. too much D. more than 17. A. thinking B. finding C. including D. taking 18. A. Have B. They C. There D. Has 19. A. When B. Where C. as D. to 20. A. from B. in C. on D. for cảm ơn ạ!