[单选题]
Equal masses of watehz2g+bm l: jqbip be7) 8wcp-++rb 2li3 lajr8w0r(r+ cn zm:xr at $20 \; ^{\circ}C$ and $80 \; ^{\circ}C$ are mixed. What is the final temperature of the mixture?
A. $40 \; ^{\circ}C$
B. $50 \; ^{\circ}C$
C. $60 \; ^{\circ}C$
D. $70 \; ^{\circ}C$