whAt soCiAl sCiEntists hAvE long FElt puzzlED ABout is why ______.
A．thE mortAlity rAtE CAn not BE prEDiCtED
B．thE DEAth toll rEmAinED stABlE yEAr AFtEr yEAr
C．A quotA For EACh typE oF DEAth hAs not ComE into BEing
D．pEoplE lost thEir livEs EvEry yEAr For this or thAt rEAson