Passage Two Questions 26 to 30 are based on the following passage. In our day of the automobile and paved highway few people ever encounter quicksand(流沙). Yet quicksand is still common in many pans of the country. It may be more erous for being less familiar. Quicksand is usually found along the shores and in the beds of rivers. It is simply sand saturated(使饱和)with water from beneath，as from a spring. The water flowing into the sand separates the grains. The suspended grains give rather easily and a heavy object placed on the suce is likely to sink. How fast it sinks depends on its weight and suce area. How does one detect quicksandIt cannot be done by the eye alone，since sand which looks firm may suddenly collapse and trap anyone who ventures out on it. The only way to be sure is to test the sand before walking on it. For test a probing(探索)pole or long stick should be used. If the pole sinks more than six inches，the sand is probably quicksand. A traveler who stumbles into quicksand will soon sink to the depth of his knees. If he stands still or struggles wildly，he will sink even further. He should at once lie on his back and stretch out his arms. Contrary to popular notion，quicksand does not objects down，and will support more weight than water alone. While the trapped person“floats”on the suce of the sand，rescuers should build a platform with boards or branches. Then they can pull him out slowly. If the trapped person is alone，he can rescue himself. When he is in the floating position，he should begin rolling toward solid ground. Rolling is the only way of getting free. It should be done with frequent rests，so that the trapped person does not tire himself. When he reaches solid ground，he should swing his legs to safety，and quickly scramble out of the quicksand.

Passage TwoThe main idea of this passage is______.