listEning 5"AstronomyClAss" {$mEDiAurl} whAt is thE DisCussion mAinly ABout A、thE DisCovEry oF thEAlphACEntAuri systEm B、thE rEAson solAr systEms ArE ConFusED with gAlAxiEs C、thE vAst ExpAnsE oF thE univErsE ArounD us D、thE moDEl At thE nAtionAlAir AnD spACE musEum