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In 1965, Mariner 4 flew by Mars, returning the first close-up images of the Red Planet. Those photos forced many scientists to believe that there had not been life on Mars. Hopes of a life-supporting Mars had been famously stoked around the turn of the 19th century by astronomer Percival Lowell, who insisted [about|for|of|on] the fact that channels on the planet were actually canals built by intelligent creatures.
The optimists got more good news in 1971 after Mariner 9 orbited Mars, becoming the first spacecraft to circle another planet. This spacecraft spotted river channels and other evidence of past liquid-water activity on the Martian surface. These discoveries helped NASA to develop two ambitious life-hunting Mars missions, Viking 1 and 2, and NASA assured the scientific community [to|about|of|on] their success. In 1975, the twin Viking landers searched [about|for|of|on] signs of microbial life in the red dirt. However, the data received by these spacecraft did not shred the convincing evidence of life on Mars. NASA apologised [for|on|about|of] their failure.
The Viking results taught NASA a valuable lesson — we did not know enough about Mars to mount a proper life hunt there. The space agency eventually embarked on a long-term "follow the water" exploration strategy, seeking to learn more about ancient environmental conditions on the Red Planet and how they changed over time. This strategy gave us many prominent Mars missions over the past few decades.
Write one number in a gap.
What have the Viking expeditions helped scientists to understand?
There is a need to better understand the planet Mars.
There was no life on Mars.
Mars had water.
Answer: [ ]