[单选题]
If the theoretical e8cl hp;c e0u5d5ts9tofficiency of a Carnot enginb y -x*k*18n9j g 3n9cjhomjna,r-iooe is to be 100%, the heat sink must 9,r1 anx- b8hgk ijjoy o-*39mnno *cjbe
A. at absolute zero.
B. at $0 ^{\circ}C$.
C. at $100 ^{\circ}C$.
D. infinitely hot.
