A、The weather. B、The corrosion of the river. C、People in search of stone as building materials. D、Bad planning. C

The oldest stone buildings in the world are the pyramids. They have stood for nearly 5,000 years, and it seems likely that they will continue to stand for thousands of years yet. There are over eighty of them scattered along the banks of the Nile, some of which are different in shape from the true pyramids.
    Some of the pyramids still look much as they must have done when they were built thousands of years ago. Most of the damage suffered by others has been at the hands of men who were looking for treasure or, more often, for stone to use in modern buildings. The dry climate of Egypt has helped to preserve the pyramids, and their very shape has made them less likely to fall into ruin. These are good reasons why they can still be seen today, but perhaps the most important is that they were planned to last forever.
    It is practically certain that plans were made for the building of the pyramids because the plans of other large works have fortunately been preserved. However, there are no writings or pictures to show us how the Egyptians planned or built the pyramids themselves. Consequently, we are only able to guess the methods used. Nevertheless, by examining the actual pyramids and various tools which have been found, archaeologists have formed a fairly clear picture of them.