In Coming to the Conclusion That Lightbeams Are Bent by Gravity

In coming to the conclusion that lightbeams are bent by gravity, Einstein reasoned that

A) accelerated observers would find that lightbeams bend, so gravity must also bend lightbeams.
B) increased mass at higher velocities causes increased gravitational effects, so lightbeams moving at lightspeed should have gravitational effects.
C) since all observers experience the same laws of physics, Newton's theory of gravity must apply to lightbeams.
D) fast- moving observers would find that lightbeams bend, so gravity must also bend lightbeams.
E) since lightspeed is the same for all observers, moving observers would observe that lightbeams bend.