下列哪项试验不是流行病学实验研究的特点（）
A、前瞻性研究，随访研究对象C
暂无解析
Iam____gratefulforthemanykindnessesyouhaveshownmyson.
Itisnotuncommonforthere_____problemsofcommunicationbetweentheoldandtheyoung.
Perhapsthecustomerhas____hismonthlystatementandnotpaidthebill.
翻译：
Bymakingvehicleslighterinweightaluminumhasgreatlyreducedtheamountoffuelneededtomovethem,(PassageTwo)
There_____nothingmorefordiscussion,themeetingcametoanendhalfanearlier.
