The "Casimir Effect" demonstrates that:
a) The quantum vacuum can exert an attractive force between two uncharged, parallel plates due to the zero-point energy of the vacuum.
b) Photons can be trapped between two mirrors, leading to a resonant energy state.
c) Particles can tunnel through a potential barrier even if they do not have enough energy to overcome it classically.
d) Gravitational waves can be produced by oscillating masses in a vacuum.