A sequential multiplexer is connected as shown in figure. Each time the multiplexer receives the clock, it switches to the next channel from 6 to 1. If the input signals are :
A = 10 cos 2p(4 x 10³t)
B = 15 cos 2p(5 x 10³t)
C = 20 cos 2p(6 x 10³t)
D = 10 cos 2p(10 x 10³t)
The minimum clock frequency should be __________ kHz.