Headache is a very common disease. The symptoms (1) ________ a headache are various. People may (2) ________ pains only one side of the head. Sometimes when the pain goes away, the head is sore. People have a headache (3) _________ they work too hard or they are too nervous about something. (4) ________ can help cure the disease but people usually have to do more than taking tablets. They can prevent headaches by changing their diets or their lifestyles or simply by going to bed. 18. A. on B. in C. of D. at 19. A. be b. have C. happen D. take 20. A. when B. but C. so D. and 21. A. Medicine B. Doctor C. Sport D. Fruit