Section B
Directions: There are 2 passages in this section．Each passage is followed by some questions or unfinished statements．For each of them there are four choices marked A, B, C and D．You should decide on the best choice.
A study published in the New England Journal of Medicine estimated that there are an average of 30 in-flight medical emergencies on U.S．flights every day．Most of them are not grave; fainting, dizziness and hyperventilation(换气过度) are the most frequent complaints．But 13% of them—roughly four a day—are serious enough to require a pilot to change course．The most common of the serious emergencies include heart trouble (46%), strokes and other neurological(神经的) problems (18%), and difficult breathing (6%).
Let's face it: plane riders are stressful．For starters, cabin pressures at high altitudes are set at roughly what they would be if you lived at 5,000 to 8,000 feet above sea level．Most people can tolerate these pressures pretty easily, but passengers with heart disease may experience chest pains as a result of the reduced amount of oxygen flowing through their blood．Low pressure can also cause the air in body cavities to expand—as much as 30%．Again, most people won't notice anything beyond mild stomach cramping．But if you've recently had an operation, your wound could open．And if a medical device has been implanted in your body—a splint, a tracheotomy(气管切开术) tube or a catheter(导管)——it could expand and cause injury.
Another common in-flight problem is deep venous thrombosis(深静脉栓)——the so-called economy-class syndrome．When you sit too long in a cramped position, the blood in our legs tends to clot．Most people just get sore calves．But blood clots, left untreated, could travel to the lungs, causing breathing difficulties and even death．Such clots are readily pred by keeping blood flowing; walk and stretch your legs when possible.
Whatever you do, don't panic．Things are looking up on the in-flight-emergency front．Doctors who come to passengers' aid used to worry about getting sued; their fears have lifted somewhat since the 1998 Aviation Medical Assistance Act gave them 'good Samaritan' protection．And thanks to more recent legislation, flights with at least one attendant are starting to install emergency medical kits with automated defibrillators(电击去颤器) to treat heart attacks.
Are you still wondering if you are healthy enough to fly? If you can walk 150 ft．or climb a flight of stairs without getting winded, you'll probably do just fine．Having a doctor close by doesn't hurt, either.
Heart disease takes up about ______ of the in-flight medical emergencies on US flights.