Physicist Fermi once proposed a famous "Fermi paradox": assuming that there is indeed an alien intelligent life in the Milky Way, considering the ancient age of the Milky Way and other factors, they should already have visited the Earth; but the fact is, when We look up at the starry sky, and what we face is the so-called "big silence"-the universe is peaceful and silent. If the following judgments are true, which one cannot explain the "Fermi Paradox"? ['We may not have reached a level where we can discover alien intelligent life.', 'Alien civilizations have progressed enough that we cannot observe them directly.', 'Alien life has created many mysterious "UFO" events on Earth.', 'Alien civilization may have reached a balance with nature and subtly integrated with the background of the galaxy.']