A disk rotates freely on a vertical axis with an angular velocity of \(30 \mathrm{rpm}\). An identical disk rotates above it in the same direction about the same axis, but without touching the lower disk, at \(20 \mathrm{rpm}\). 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. \(50 \mathrm{rpm}\)

B. \(40 \mathrm{rpm}\)

C. \(25 \mathrm{rpm}\)

D. \(20 \mathrm{rpm}\)

E. \(10 \mathrm{rpm}\)