回答下面的题目:
Exercise Being Good or Bad
Can exercise be a bad thing? Sudden death during or soon after strenuous exertion on the squash court or on the army training grounds, is not unheard of. 51 trained marathon runners are not immune to fatal heart attacks. But no one knows just 52 common these sudden deaths linked to exercise are. The registration and
investigation of such 53 is very patchy; only a national survey could determine the true 54 of sudden deaths in sports. But the climate of medical opinion is shifting in 55 of exercise, for the person recovering from a heart attack as 56 as the average lazy individual. Training can help the victim of a heart attack by
lowering the 57 of oxygen the heart needs at any given level of work 58 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, 59 .particular, be screened for signs of heart disease before 60 vigorous exercise?
Most cases of sudden death in sport are caused by lethal arrhythmias in the beating of the heart, often in people 61 undiagnosed coronary heart disease. In North America 62 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, 63 , 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 64 at risk of developing coronary heart disease. Individuals showing a particular abnormality in their ECGs 65 , they say, a 10 to 20 times greater risk of subsequently developing signs of coronary heart disease, or of sudden death.
第51题应选：

A . having
B . had
C . having been
D . have

参考答案：D

参考解析：

【解析】根据空格后时态可知，此处缺少一个一般现在时态的谓语动词，根据语法和句意，D项符合句意。