In the 1960s and 1970s, the greatest fear was [1]that the human race, and possibly all advanced② life forms on the planet, could be wiped③ out by nuclear④ missiles, just at the push of a button⑤.Today, however, environmental problems have taken over as the greatest risk to life on Earth.Scientists are thinking of ways to lower this risk, such as replacing coal and oil with forms of renewable energy⑥.But they are also preparing for the worst：what can we do if the terrifying⑦ scenes in films such as The Day After Tomorrow happen in real life? What is our Plan B for Earth?
[1]that引导表语从句。
One option is [2]to explore other planets to see if we could live on them.The most likely choice is Mars, [3]which is relatively⑧ close to Earth and has an environment less hostile⑨ than that of other planets.Mars has fascinated⑩ people since ancient times, and today our interest in Martian exploration is greater than ever before.Films such as The Martian enjoy worldwide popularity.More governments and organisations are making efforts? to educate the public on the Red Planet, for example, the Mars Desert Research Station in the Utah desert of the US and the Mars Village in Northwest China's Qinghai Province.
[2]动词不定式短语to explore other planets作表语；动词不定式短语to see if we could live on them作目的状语；if引导宾语从句。
[3]which引导非限制性定语从句，修饰名词Mars。
Since the 1960s, we have been sending unmanned? spacecraft? to Mars.Our probes have orbited? the planet, [4]sending back valuable data and stunning? images.Our robots have explored the Martian surface, [5]testing the soil and searching for? resources, water and signs? of life.[6]Encouraged by discoveries over the years, space agencies? of various countries are planning manned missions to Mars [7]that could take place within the next 25 years.
