For an amorphous polymer, its glass transition temperature () indicates which of the following:

Select all that apply and explain a. A discrete temperature at which the polymer undergoes an abrupt transition in its mechanical behavior. Below , the polymer behaves as a glass. Above the polymer is rubbery. b. A discrete temperature representing the mid-point in the temperature range over which the polymer undergoes a transition in its mechanical behavior. Below , the polymer behaves as a glass. Above , the polymer is rubbery. c. A discrete temperature representing the mid-point in the temperature range over which the polymer undergoes a transition in its mechanical behavior. Above , the polymer behaves as a glass. Below , the polymer is rubbery. d. A discrete temperature at which the polymer molecules undergo a reversible transformation to an amorphous, glass-like structure.