The total amount of blood accounts for about 7% ~ 8% of body weight. If a normal adult weighs 60 kg, his blood volume is about 4200 ~ 4800 ml. When the adult's blood loss is less than 400 ml, the blood loss of 1 time does not exceed 10% of the total blood volume, so there is no obvious symptom and blood transfusion is not needed. A healthy adult donates about 200 ~ at a time. When the blood loss reaches 800- 1000 ml, that is, 20%, there will be symptoms such as pale face, pale lips, cold sweat on the skin, cold hands and feet, shortness of breath, rapid and weak pulse, and blood transfusion can be performed; When the blood loss exceeds 1200ml, that is, 30%, it will be life-threatening; When the blood loss reaches more than1500ml, that is, about 35%, it can cause insufficient blood supply to the brain, and the injured person will have symptoms such as blurred vision, thirst, dizziness, confusion or anxiety, and even coma, and must be transfused immediately.
So choose: B.