A、It remains unknown yet. B、Around five thousand years ago. C、About two million years ago. D、About three hundred thousand years

Pyramids along the Nile River are the most interesting buildings in the world. Thousands of years ago in Egypt, kings built the pyramids. They used to build them as their tombs. The kings thought the pyramids would help them find life after death. They also wanted the world to remember them as important people.
    The oldest pyramid we know today is the one near Sahara in Egypt. It is about 5,000 years old. The largest is the pyramid of Khufu. It is made of two million three hundred thousand huge stones, most of them higher than a person. It is about 114 meters high. To build the pyramid of Khufu, 100,000 men worked for twenty years. We know there were wonderful treasures in the pyramid. Robbers went into the pyramids and took many of these treasures. Today some of the treasures are in museums though.
    How did the people of ancient days build the pyramids? How did they carry and lift the huge stones? Each stone fits so well and they didn’ t have our modern machines.  Scientists have studied the pyramids but no one can say for sure how they did it.