Read the passage then choose the best answer to each question that follows. Identify your answer by writing the corresponding letter A, B, C or D on your answer sheet.

The symptoms of hay fever include watery and itchy eyes and a runny, congested nose. People suffering from hay fever may experience occasional wheezing and repeated bouts of sneezing and may even lose their sense of smell. Some victims of hay fever may also have stopped-up ears. About 30 percent of those who suffer from hay fever may develop the symptoms associated with periodic asthma or a sinus infection. The allergen-antibody theory does not fully explain allergic reactions because the membranes and glands in eyes and ears are controlled by the independent nervous system, which keeps these organs in balance. But the independent nervous system itself is part of the emotional- response center and may cause the feelings of anger, fear, resentment, and lack of self-confidence in reaction to allergy-causing substances. The most common cause of hay fever is the pollen of ragweed, which blossoms during the summer and autumn. When airborne pollen particles, as well as mold, come into contact with the victim’s membranes, they can cause allergic reactions that release histamine and result in virtual blockage of air passages. To prevent hay fever or to decrease the severity of its symptoms, contact with the ragweed pollen should be reduced. Although some communities have attempted to eliminate the plants that cause the reactions, elimination programs have not been successful because airborne pollen can travel considerable distances. Antihistamine can help with short but severe attacks. Over extended periods of time, however, patients are prescribed a series of injections of the substance to which they are sensitive in order to increase immunity and thus be relieved of the seasonal allergy. (Resource: Toefl test strategies by Eli Hinkel, Ph.D)

Which of the following is NOT mentioned in the passage as a cause of allergies?