Given the fact "if an organism becomes too hot then that organism may die", what is the answer to the question or completion "A frog was sick because it wasn't staying warm enough."

OPTIONS: (I) The frog was getting burned by a heat lamp; (II) The water in the tank had dried up.; (III) the frog was unable to regulate its body temperature without an external heat source; (IV) The frog was using too much ice in its drinks;
(III)