Analysis of the composition of quicksand shows that there are four key ingredients—sand, obviously water, clay and salt. Together these materials form a structure resembling a house of cards, with large water-filled gaps between the sand particles, which are loosely glued in place by the clay. As long as it’s left alone, the structure remains stable. But as soon as it’s disturbed, by stepping on it, the clay changes from a jelly-like consistency to a runny liquid. The effect is the same as stirring a pot of yoghurt. Liquefying(溶解)the clay makes the quicksand about one million times runnier, and the whole house of cards comes tumbling down, with you inside it.
Very quickly, the sand sinks to the bottom and the water floats to the top. This is where the salt comes in. When there’s enough salt present, as soon as the clay particles liquefy, electrical charges make them begin to stick together to form bigger particles and these also settle with the sand. Quicksand is a mixture which looks like solid, but behaves more like a liquid.
Despite its murderous reputation, quicksand does not such people under and swallow them, although it can hold a person in firm grip. The human body is more buoyant(有浮力) in quicksand than in water, and sooner or later anyone trapped in it will float.
Unlike most liquids, quicksand’s viscosity, or “runniness” can suddenly change if it comes under pressure, for example under a human foot. The surface gives way and the victim quickly sinks in up to the knees, surrounded by an area of dirty things that turns semi-solid around its victim. Escaping from the quicksand’s grip(紧握) requires a large amount of force. Without something solid to pull at, people often find they are stuck fast. Pulling at one leg simply makes the other one sink further.
So how do you escape from quicksand’s control? Stay still and call for help. Staying still stops you sinking any further, until--with luck--help arrives on the scene. If no one appears and you need to draw yourself out, gently lie down on your back until your body is floating on the sand. Next, roll over onto your stomach and pull yourself forwards with your hands, so that you gradually “swim” towards firmer ground. It is a slow and dirty business, but it works.
This technique has one barrier: it goes against a very powerful human nature. Faced with danger in the open, nine out of ten stay on their feet so that they can run away, in which case it can worsen the situation.
Which ingredient of the quicksand make someone trapped in it stuck and stuck fast?

A．Sand

B．Clay

C．Water

D．Salt

From the passage we can learn that ________

A．as long as you are strong enough, you can pull yourself up from quicksand on your own

B．clay glues sand particles loosely, which makes quicksand stable, unless it is disturbed

C．because anyone trapped in quicksand will float, there is little possibility of him dying

D．looking like solid ground, quicksand is actually liquid

If someone is trapped in quicksand, what should he do?

A．Remain calm and wave for help

B．Stand in quicksand and get ready to run away as quickly as possible

C．“Swimming” is one of the best choices he can use to escape

D．Once in quicksand, lie down on his stomach immediately and swim slowly to safety

What might be the best title of the passage?

A．A human killer--quicksand

B．How to escape from danger

C．An awful experience in quicksand

D．What to do if trapped in quicksand

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