A disk rotates freely on a vertical axis with an angular velocity of 80 rpm . An identical disk rotates above it in the same direction about the same axis, but without touching the lower disk, at 20 rpm . The upper disk then drops onto the lower disk. After a short time, because of friction, they rotate together. The final angular velocity of the disks is:______
A. 50 rpm
B. 40 rpm
C. 25 rpm
D. 20 rpm
E. 10 rpm