A、It changes from wet to dry about every 20000 years. B、It was an ocean about 20000 years ago. C、It has been gradually enlarging

A new study suggests climate conditions in the Sahara Desert have changed from wet to dry about every 20000 years. The study was carried out by scientists at America’s Massachusetts Institute of Technology, or MIT. Findings were published in the journal Science Advances. The research was partly funded by the National Science Foundation.
    The Sahara Desert is known as one of the hottest, driest and most desolate areas in the world. But previous evidence has suggested the Sahara did not always experience such extreme heat and dry conditions. The evidence included material collected from fossils and rock paintings from the area. The evidence showed that at times, the Sahara changed to a very wet climate. This permitted plants and animals to develop and grow and led to the creation of human settlements. Now, researchers have discovered more evidence of this changing climate. The scientists examined dust gathered from the coast of West Africa over the past 240000 years. The research showed that during that period, the Sahara’s climate kept changing between wet and dry about every 20000 years.
    David McGee is an associate professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences. He helped lead the study. McGee said the new evidence supports the idea that the area’s climate repeatedly kept changing over the years. " Our results suggest the story of North African climate is dominantly this 20000-year beat, going back and forth between a green and dry Sahara, " he said. McGee and other researchers reported these back and forth climate changes were mainly driven by changes in the Earth’s axis as the planet orbits the sun. This process affects the amount of sunlight between seasons. The research suggests that every 20000 years, the Earth receives more intense summer sunlight. When the Earth’s axis changes again, the amount of sunlight is reduced. This season change happened continuously every 20000 years, the study found. The other part of the season produced monsoon conditions, resulting in a wetter, greener, plant-rich environment. When the rainy activity weakens, the climate becomes hot and dry, like the Sahara remains today. The scientists based their research on dust samples collected from ocean sediment. McGee says he thinks the latest research can be valuable in studying the Sahara’s history as it relates to human settlement. "What times could have been good for humans to settle the Sahara Desert and cross it to disperse out of Africa, versus times that would be inhospitable like today, " he said.
    Questions 19 to 22 are based on the recording you have just heard.
    19. What does the new MIT research find about the Sahara Desert?
    20. What was the previous evidence showing that the Sahara did not always experience extre conditions?
    21. What caused the climate changes in the Sahara Desert?
    22. Why does McGee believe that the latest research is valuable?