填空题

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 (36) , but he is mercilessly (37) He cannot escape. Slowly he sinks deeper and deeper until at last he is gone, (38) in the treacherous earth. The solid ground was solid only in (39) It was actually quicksand, which is a deep (40) of fine sand mixed with water.
How is quicksand formed Water pushes up from below the surface and is held by the sand. The (41) 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 (42) of quicksand.
Is it true that a man who steps into quicksand is (43) to die No, four men have fought their way from quicksand to firm land again. (44) . Quick movements will make the sand yield temporarily, but then it rushes and settles solidly around the body.
(45) . 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, (46) .