(b)
Given that:
velocity of blocks \(=10\,ms ^{-1}\)
Friction force between \(B\) and \(A=20\,N\).
As we know,
Force \(=\) mass \(\times\) acceloration
So, Friction force between the block \(B\) and the ground is zero as acceleration of the blocks is zero as they are moving with constant velocity.