specimen is a rigid as a board and flexible elastomer but they are both thermosets. what makes the difference in their mechanical behaviour

Demonstration: the specimens shown are (a) a piece of cured epoxy resin, (b) a rubber band, and (c) a strip of linear low density polyethylene (LDPE). Answer the followings: Specimen (a) is a rigid as a board and (b) is a very flexible elastomer, but they are both thermosets. What makes the difference in their mechanical behavior? (a) * Upon drawing/pulling, specimens (b) and (c) will both exhibit high levels of elongation. However, when the deforming force is removed, specimen (b) will instantly retract to its original shape and form, but specimen (c) will remain permanently deformed. Why? (b) c