疾病时间分布的表现形式有：①短期波动、②季节性、③（）、④（）。
参考答案：周期性；长期趋势
抗心律失常药物的分类?
Yesterday the traffic was _______ for hours by a terrible accident on this stretch of the motorway. [ ]
A. held back
B. held on
C. help up
D. held out
