Choose words from the article to complete this summary.Mars as a planet is not 1________. There is not enough oxygen to breathe. There is no 2________, just ice, and it can get very cold there. Some scientists, however, believe Mars can be terraformed. Perhaps factories could be built to increase the levels of 3_______. This would change the atmosphere on Mars. But terraforming Mars would take many centuries and would be expensive.The first step would be to send 4_________ missions there. Then homes could be built. Enormous 5__________ would be required, first for plants, then for people. Perhaps 6_______ or wind turbines could be used for power. But even after 1,000 years, people might need to wear equipment similar to 7________ in order to breathe.