By careful observation and precise calculation, Nicolaus Copernicus _____ that there is no one center in the universe and the sun is the center of the solar system.
______ the aids from the outside, the positive attitude of the disaster-stricken people _____ the key role in the reconstruction of their destroyed towns and villages.
After several years of observation, the research group ______ the conclusion that language acquisition is faster when students ______ to the situation where it is spoken.