quEstions 14 to 16 ArE BAsED on A tAlk on thE worlD’s Air pollution. you now hAvE 15 sEConDs to rEAD quEstions 14 to 16.

whAt Do thomAs kArl AnD kEvin trEnBErth BEliEvE As to Air pollutionA．ClimAtE ChAngE CoulD BE CAusED By mostly nAturAl ForCEs.
B．toDAy grEEnhousE gAsEs ArE thE lArgEst humAn inFluEnCE on gloBAl ClimAtE.
ClimAtE ChAngE is truly A gloBAl issuE, But it CAn’t provE to BE thE grEAtEst ChAllEngE to us.
D．CArBon DioxiDE lEvEls in thE AtmosphErE hAvE FAllEn By 31 pErCEnt sinCE prEinDustriAl timEs.