题目

In many countries, heart disease kills a lot of women and even more men. There are four major causes of this disease and these are smoking, poor or bad diet, a family history of heart disease and stressful work. Among these main wrong-doers, the most common cause is stress.     What is stress and how do you feel when you are under stress? Imagine you live alone – you are lying in bed at night and, suddenly, you hear someone in the kitchen. Immediately your heart begins to beat very fast and you feel that you can hardly breathe. What you are felling is not stress, but fear. Howeve动物细胞分裂时，变化最明显的部位是 [     ]A．细胞膜 B．细胞质C．细胞核 D．细胞