A Mixture of Hydrogen and Chlorine Remains Unreacted Until It $\Delta$

A mixture of hydrogen and chlorine remains unreacted until it is exposed to ultraviolet light from a burning magnesium strip. Then the following reaction occurs very rapidly. Select the statement below that best explains this behavior.

A) The reactants are thermodynamically more stable than the products.
B) The reaction is spontaneous, but the reactants are kinetically stable.
C) The reaction has a small equilibrium constant.
D) The ultraviolet light raises the temperature of the system and makes the reaction more favorable.
E) The negative value for $\Delta$ S slows down the reaction.