thE phrAsE "hEAt rECovEry" rEFErs to A ______.
A．mEthoD oF ConCEAling thE sourCE oF hEAt
B．spECiAl Form oF Air ConDitioning
C．supplEmEntAry hot wAtEr systEm
D．wAy oF rEClAiming AnD rE—using hEAt