英语句法问题
Arectalstudy,publishedinlastweek'sJournaloftheAmericanMedicalAssociation,offersapictureofhowriskyitistogetaliftfromateenagedriver.这句话中‘howriskyitistogetaliftfromateenagedriver’做的是什么成分?是定语还是补语?麻烦再附带分析,