流行病学研究的主要问题是
A．疾病分布及影响分布的因素
B．疾病的病因
C．疾病的防制措施
D．流行病研究方法
E．以上均是
参考答案：E
He is ______ as a leader but he hasn't ______ in teaching.
A．success; many experiences
B．a success; much experience
C．great success; an experience
D．a great success; an experience
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