Why is energy conserved here?

Question

Two identical discs of mass M and radius R are connected by a light rod. The assembly rests at a corner, the vertical wall is smooth and there is sufficient friction on the floor to ensure pure rolling of disc B. The system starts from position θ=0.

This is our scenario and I've been told energy is conserved in this scenario and I don't understand why that is.
Aren't the normal forces from the wall and the floor external forces ?
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Protein · Accepted Answer

Aren't the normal forces from the wall and the floor external forces

Yes they are but the work done by the mentioned forces is zero. The important concept here is work done by friction when object is rolling .
Look at normal forces acting at both points of contact and the direction of motion of respective disks. What do you observe? They are perpendicular! Thus the work done is zero.
look here for work by frictional force in rolling motion.

Why is energy conserved here?

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