[单选题]
If the theoretical efficia35b(cwm+q xf3x* w jb: 1gh gfr2;eovency of a Carnot engine is jknvd5 mzf.fw; fc1/t6r+,obto be 100%, the heat sink must be1/nmvkf;, .cwzt d +b6f5 jrof
A. at absolute zero.
B. at $0 ^{\circ}C$.
C. at $100 ^{\circ}C$.
D. infinitely hot.
