Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 9 to 13.

Scientists have experimented with a new procedure for alleviating the damage caused by strokes. Strokes are frequently caused by a blood clot lodging in the tree of arteries in the head, choking the flow of blood. Some brain cells die as a direct result of the stroke, but others also die over several hours because the proteins spilling out of the first cells that die trigger a chemical chain reaction that kills the neighboring cells.

The current method of reducing the amount of damage is to give a clot dissolver, known as TPA, as soon as possible. But generally TPA is not given to the patient until he or she reaches the hospital, and it still does not immediately stop the damage.

The new technology, still in the research stage, involves chilling the area or the entire patient. It is already known that when an organ is cooled, damage is slowed. This is why sometimes a person who has fallen into an icy pond is not significantly harmed after being warmed up again. The biggest issue is the method of cooling. It is not feasible to chill the head alone. Doctors have chilled the entire body by wrapping the patient in cold materials, but extreme shivering was a problem.

The new idea is to cool the patient from the inside out. Several companies are studying the use of cold-tipped catheters, inserted into the artery in the groin and threaded up to the inferior vena cava, which is a large vein that supplies blood to the abdomen. The catheter is expected to cool the blood that flows over it, thus allowing cooler blood to reach the area of the stroke damage.

It is not expected that the cooling will be substantial, but even a slight decrease in temperature is thought to be helpful. In effect, the patient is given a kind of forced hypothermia. And doctors believe it is important to keep the patient awake so that they can converse with the patient in order to ascertain mental condition.

Studies continue to determine the most effective and least damaging means of cooling the patient in order to reduce this damage.

According to the passage, what causes a stroke?

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