The famous Dr. Edward Jenner was busy trying to solve the problem of smallpox. After study

ing case after case, he still found no possible cure. He had reached an impasse in his thinking. At this point, he changed his tactics. Instead of focusing on people who had smallpox, he switched his attention to people who did not have smallpox. It turned out that dairymaids(挤奶女工)apparently never got the disease. From the discovery that harmless cowpox gave protection against deadly smallpox came vaccination(牛痘) and the end of smallpox as a scourge(灾祸) in the Western world.

We often reach an impasse in our thinking. We are looking at a problem and trying to solve it and it seems there is a deadened, and "aporia" (the technical term in logic meaning "no opening"). It is on these occasions that we become tense, we feel pressured, overwhelmed, in a state of stress. We struggle vainly, fighting to solve the problem. Dr. Jenner, however, did something about this situation. He stopped fighting the problem and simply changed his point of view--from patients to dairymaids, picture the process going something tike this: Suppose the brain is a computer. This computer has absorbed into its memory bank all your history, your experiences, your training, your information received, through life, and it is programmed according to all this data. To change your point of view, you must reprogram your computer, thus freeing yourself to take in new ideas and develop new ways of looking at things. Dr. Jenner, in effect, by reprogramming his computer, erased the old way of looking at his smallpox problem and was free to receive new alternatives.

What does "impasse" probably mean？

A．deadened

B．depression

C．solution

D．peak