5、t12.I can hardly concentrate _____ my work ______ the loud noise outside. A.on; with B.in; for C.on;forD.with; with13.Nobodyknowsforsurehowdinosaursdiedout65,000,000yearsago.It_____becauseofsuddenchangesoftheirlivingconditions.A.musthavehappenedB.wouldhavehappenedC.shouldhavehappened