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Researchers in the U.S. (are discovering, have discovered, were discovered, have been discovered) a powerful drug, pregnenolone, that can improve memory nice. During the experiment in the laboratory, mice (were trained, had been trained, trained, were training)… to associate a sound with a shock a few seconds later. They had to remember their way through a maze to avoid to shock. Then some of the mice were injected with the drug. These mice (showed, were shown, have been shown, are showing) …such a significant improvement in memory that the researches (were being astonished, astonished, were astonished, had been astonished) … Scientists (knew, had known, had been knowing, have known) … when they stared the experiment that pregnenolone can help people recover after car crashes, but what this new research shows is that it (is played, has been played, played, plays) … an even more important role. The results suggest that it may one day be possible to treat people whose memories (affected, had affected, have affected, have been affected)… by old age or disease. The drug (has already been tried, has already tried, is already trying, had already trying)… out on a number of patients badly affected by Alzheimer’s disease and incapable of recognizing members of their own family. Unfortunately, they only showed a small improvement, if any, but the scientists (believe, were being believed, are believing, are believed)… that the doses they used might have been either too high or too low. They are confident that they ( will be reached, will reach, had reached, have been reached) their long-term aim of producing a drug that will be safe and effective for humans.