在研究疾病流行时,首先应用流行病学的方法是
A.制定控制措施
B.确定是否存在流行
C.分析疾病的病因
D.描述疾病的三间分布特点
E.明确治疗手段
参考答案:D
Not until _________ did man accept that fact that all living things must die.
A.did many years pass
B.many years past
C.many years had passed
D.many years have passed
总磷与有效磷