[单选题]
If the theoretical effb kxm: .d;d5 ;qygx9ffird :gzfd o/lapasfbf ;81 40z1jfd 65uciency of a Carnot engine is to be 100%, the heat sink must slf :bfazd1/jo fa;16zfru5 4d8p0 dgbe
A. at absolute zero.
B. at $0 ^{\circ}C$.
C. at $100 ^{\circ}C$.
D. infinitely hot.
