单选题 .  There is never a good time to have a heart attack, but the wise person afflicted with clogging arteries (动脉堵塞) might want to be especially careful in future to avoid stress and watch the diet as August rolls around.
    The NHS, it is revealed today, has its very own black Wednesday, when death rates go up by an average of 6%; and there is a somewhat disturbing underlying cause—the arrival on the wards of a new intake of junior doctors.
    On the first Wednesday in August every year, a freshly qualified set of junior doctors arrives on the wards. Pristine (质朴的) and eager and brilliant they no doubt are, but while they are finding their way around something unexplained and slightly perplexing appears to happen.
    Researchers from the Dr. Foster unit and the department of acute medicine at Imperial College London say there has been a suspicion for years that more people die on the day the new doctors arrive, but for the first time they have established that it happens—although they say the rise in deaths is very small.
    They do not blame the doctors' inexperience or confusion in the hospital and say it is also possible that only the severest cases are admitted in that week, because of the changeover.
    Their study has international implications, the researchers say. "A similar effect has been recorded in the US (known as the 'July phenomenon')," they write in their paper. But previous studies have looked only at a few hospitals.
    The Imperial study is far bigger, examining carefully data from nearly 300,000 patients in 175 hospital trusts. It compared death rates on the first Wednesday in August with the last Wednesday in July. The difference was most marked in medical cases, where there was an 8% increase in deaths; there was no difference in surgical cases.
    "We wanted to find out whether mortality rates changed on the first Wednesday in August, when junior doctors take up their new posts," said senior author Paul Aylin. "What we have found looks like an interesting pattern and we would now like to look at this in more detail to find out what might be causing the increase."
    "Our study does not mean that people should avoid going into hospital that week. This is a relatively small difference in mortality rates, and the numbers of excess deaths are very low. It's too early to say what might be causing it."
    Shree Datta, chair of the British Medical Association's junior doctor committee, said the study had to be judged alongside others looking at mortality rates before and after junior doctors start their new jobs, but added: "Clearly even a small increase in death rates is of great concern and we need further research to see whether this is a general effect or an anomaly (反常)."1.  According to the passage, August is the time when ______.