【B1】 【B2】

A man steps on what seems like solid ground but discovers with horror that the ground is giving way beneath his feet. The man struggles desperately, but he is mercilessly trapped He cannot escape. Slowly he sinks deeper and deeper until at last he is gone, buried in the treacherous earth. The solid ground was solid only in appearance It was actually quicksand, which is a deep mass of fine sand mixed with water.
     How is quicksand formed? Water pushes up from below the surface and is held by the sand. The grains of sand are forced apart by the water. They cannot hold any weight. The subsurface water may have come from a spring, a river, or a stream. Sometimes pools of water near beaches become filled with sand. When the soil under these pools does not allow for good drainage, the sand can become stretches of quicksand.
   Is it true that a man who steps into quicksand is doomed to die? No, four men have fought their way from quicksand to firm land again. It is panic that creates the condition that can result in death．for the more a person struggles，the worse matters become. Quick movements will make the sand yield temporarily, but then it rushes and settles solidly around the body.
     A man trapped in quicksand should either lie back，or not move at all，except to stretch out his arms. When the weight of the sand his body has displaced equals his weight, he will stop sinking. With feet held still, and slow movements of arms, as in the backstroke in swimming men have managed to roll to safety and reach firm ground.