单选题 Everyone has suffered from headaches, but 21 recently medical researchers were not certain what caused them. Blew research is giving us more 22 about two common types of headaches: tension headaches and migraine headaches. The most painful 23 are migraines, which 24 about eight to ten percent of the time and are 25 as a very sharp, throbbing pain. Most headaches, 26 , are tension headaches. They are unpleasant, of course, but not as painful as the migraine 27 . Usually they are experienced as a 28 ache on both sides of the head or in the back or forehead. These headaches are caused by the tightening of the muscles of the head and neck, which 29 causes the blood vessels to narrow, making it difficult for the brain to receive the oxygen (water, nutrition, blood) it needs. This is what causes the pain. Migraines, 30 , result from the blood vessels in the brain enlarging, causing swelling in the brain. This swelling results in terrible pain. The headache may last for a day or two and 31 the individual sick and weak. Migraines are apparently 32 , because several individuals in the same family usually have them. Doctors believe that the brains of these individuals 33 in unusual ways to small problems, like failing a test or eating unwisely. 34 there are warning. 35 before a migraine occurs, such as spots appearing before the eyes or a sick feeling in the 36 . If the person takes medicine or caffeine as soon as he or she begins to feel the headache coming 37 , it can be 38 Caffeine and the medicine, called ergot, cause the blood vessels to narrow. Once the headache 39 , sleep and a cool 40 on the head may help.