Can exercise be a bad thing？ Sudden death during or soon alter strenuous exertion on the s

quash court or on the army training grounds, is not unheard of【41】trained marathon runners are not immune to fatal heart attacks. But no one knows just【42】common these sudden deaths linked to exercise are. The registration and investigation of such【43】is very patchy; only a national survey could determine the true【44】of sudden deaths in sports. But the climate of medical opinion is shifting in【45】of exercise, for the person recovering from a heart attack as【46】as the average lazy individual. Training can help the victim of a heart attack by lowering the【47】of oxygen the heart needs at any given level of work【48】the patient can do more before reaching the point where chest pains indicate a heart starved of oxygen. The question is, should middle-aged people,【49】particular, be screened for signs of heart disease before【50】vigorous exercise？

Most cases of sudden death in sport are caused by lethal arrhythmias in the beating of the heart, often in people【51】undiagnosed coronary heart disease. In North America【52】over 35 is advised to have a physical check-up and even an exercise electrocardiogram. The British, on the whole, think all this testing is unnecessary. Not many people die from exercise,【53】, and ECGs (心电图) are notoriously inaccurate. However, two medical cardiologists at the Victoria Infirmary in Glasgow, advocate screening by exercise ECG for people over 40, or younger people【54】at risk of developing coronary heart disease. Individuals showing a particular abnormality in their ECGs【55】, they say, a 10 to 20 times greater risk of subsequently developing signs of coronary heart disease, or of sudden death.

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A．Then

B．Though

C．Since

D．Even