Fresh fruit and vegetables _______ long on hot summer days. [ ]

A. can't be kept  

C. don't keep    

C. won't be kept  

D. are not kept

临床工作中，流行病学方法可用于

A．探讨某种疾病的病因及危险因素
B．筛检某种疾病的病人
C．对诊断试验或方法进行评价
D．评价治疗效果与预后
E．以上均是