A、A sand beach. B、A deep valley. C、A high mountain. D、A grain of sand. D

Here is a story. A participant in the long-distance race got his shoes filled with sand when he was crossing a beach. He had to stop to get the sand out hastily before he resumed running. Unfortunately a grain of sand remained rubbing the sole so that his steps became increasingly painful. Reluctant to halt and get rid of the sand, he continued to run in spite of the pain until he could stand no more. He dropped out of the contest just a few yards from the finishing line. As he managed to get out of the shoe painfully, he was surprised to find the cause of his lasting pain was only a grain of sand.
    It seems that the greatest obstacle on one’s way forward may not be a high mountain or a deep valley but a grain of sand that is hardly visible. To avoid blame on a minor fault one may tell a lie. That adds a burden to a heavy heart and weighs it down. In the days to come he will have to make up one story after another to cover the lie he told and the fault he committed. Thus he will never be able to free himself from lasting anxiety, worry and regret, to the ignorance that all his sufferings come from only a grain of sand -- the first lie he told.