[单选题]
If the theoretical efficiency7 roim(gvhjqapn1v251 hx 5 e) of a Carnot engine is to be 100%, the xc o. 3z+q53p9jp8x mnz4 sjidheat sink mu953 ojxznp zxdismqcp.+j 483st be
A. at absolute zero.
B. at $0 ^{\circ}C$.
C. at $100 ^{\circ}C$.
D. infinitely hot.
