Culture shock is the result of the removal of the familiar. Suddenly the individual is faced with the necessity of working, commuting, studying, eating, shopping, relaxing, even sleeping, in an unfamiliar environment organized according to unknown rules. In mild form, culture shock shows itself in symptoms(症状) of fatigue , irritability(易怒) and impatience. Being unable to interpret the situations in which they find themselves, people often believe they are being deliberately deceived or exploited by host -country nationals. They tend to perceive (感觉，觉察到) rudeness where none is intended. Their efficiency and flexibility is often impaired and both work and family suffer. Some people may respond by developing negative stereotypes (老一套)of the host culture, by refusing to learn the language and by mixing exclusively (排外地，专有地) with people of their own cultural background. In extreme cases, rejection may be so complete that the individual returns immediately to their own culture, regardless of the cost in social, economic or personal terms. Alternatively, people may retreat into their own private world, either mentally or physically.People have culture shock because______.
A．they are unfamiliar with new environmentB．they are treated badlyC．they must study hardD．they don"t like the foreigners