Read the text and fill in the gaps with the correct present tense of the verbs given in brackets.
Unsolved Mysteries
Humans 1) _______ \(learn\) many things over the years. For example, we 2) _______ \(discover\) DNA, and we 3) _______ \(cure\) many diseases. We 4) ______ \(visit\) the Moon and we 5) _______ \(send\) robots to Mars. However, scientists 6) ______ \(not, understand\) everything about the universe. They 7) ______ \(not, know\) if it 8) ______ \(get\) larger or smaller. Despite the fact that scientists 9) ______ \(look for\) extraterrestrial intelligence for many years, they still can’t say for sure if we 10) ______ \(be\) alone in the universe or there 11) ______ \(be\) other intelligent life out there.
Although scientists 12) ______ \(not, agree\) about everything, there 13) ______ \(be\) some facts about the universe that they are sure about. Take planet Venus, for instance. Here the temperature 14) ______ \(reach\) over 400 degrees Celsius. And another unusual fact is that a day on Venus 15) ______ \(last\) longer than a year. Many scientists 16) ______ \(still, search for\) life in other parts of the universe. But 17) ______ \(look for, they\) life on Venus? It 18) ______ \(not, seem\) to be the kind of place where life can exist. The atmosphere of Venus 19) ______ \(consist\) mainly of carbon dioxide. And it 20) ______ \(have\) an air pressure 90 times greater than that of Earth.
Scientists 21) ______ \(not, discover\) all the secrets on our planet either. For example, how 22) ______ \(travel, migrating birds\) thousands of miles over land and sea without getting lost? There 23) are \(be\) some theories. One theory 24) ______ \(be\) that birds have magnetic particles in their brains to help them find their way. However, science 25) ______ \(not, prove\) it yet.
One more mystery is earthquake lights. Blue and white lights flashing 26) ______ \(occur\) in the sky just before an earthquake. People 27) ______ \(report\) this for many years. Recently taken photographs 28) ______ \(prove\) that the lights exist, but researchers 29) ______ \(not, find\) the cause so far. They 30) ______ \(still, work\) on solving this mystery.
The human body 31) ______ \(contain\) many secrets as well. Why 32) ______ \(yawn, we\), for example? And why is yawning so “contagious”? We 33) ______ \(yawn\) when other people yawn. In fact, maybe you 34) ______ \(yawn\) right now while reading this sentence.
These and other mysteries 35) ______ \(be\) a challenge for scientists. Although we 36) ______ \(not, solve\) them so far, we 37) ______ \(make\) progress every day.