6、rtwoyearsandahalfC.sincetwoandahalfyearsD.sincetwoyearsandahalf()16.Myfriendstoldme_______somepolicemeninmyroomjustnow.A.theysawB.toseeC.theyhaveseenD.theyseeing()17.It'sourduty_______theclassroom_____.A.keep;cleanB.tokeep;cleanC.tokeep;cleanerD.keep;cleaner()18.I_______ m