A、More than forty millions. B、More than four millions. C、Less than four millions. D、Less than forty millions. B新闻开头提到的是对涉及400多万患

(5) A review of more than 30 studies involving more than four million patients suggest that (6) having an irregular heart beat poses a greater risk to women than to men. It found that women with Atrial Fibrillation, or AF, were almost twice as likely to have fatal heart disease and strokes. The scientists from Oxford University said (7) one possible cause was that women were being diagnosed later than men. Another was that they may respond less well to drugs to combat AF. June Davison, from the British Heart Foundation, said AF was under-diagnosed in both men and women. "It is important that healthcare services for the prevention and treatment of AF take into account the different effects of gender on the condition," she said. "More research is needed to find out more about the underlying causes of these differences."
5. How many people are involved in these studies?
6. What do these studies find?
7. Which one of the following is the possible cause according to the studies?