thE primAry purposE oF thE pAssAgE is to______.
A．AlErt rEADErs to thE ExistEnCE oF hDr's As An AvAilABlE EnErgy sourCE
B．DoCumEnt thE ChAllEngEs thAt hAvE BEEn surmountED in thE EFFort to rECovEr EnErgy From hDr's
C．wArn thE usErs oF CoAl AnD oil thAt hDr's ArE not An EConomiCAlly FEAsiBlE AltErnAtivE
D．EnCourAgE thE usE oF nEw tEChniquEs For thE rECovEry oF EnErgy From unDErgrounD hot wAtEr AnD stEAm