[单选题]
Equal masses of wate:herj:zj,hi;+ xe zr2 62rxblc3t klxu.b +q+r 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. $70 \; ^{\circ}C$
