Which of the following pairs belong to the category of cold blooded animals?

Which of the following pairs belong to the category of cold blooded animals?

A. Snakes and birds
B. Bat and rats
C. Frog and snakes
D. Birds and monkey

The answer is the pair of frogs and snake are both cold blooded animals. Both these animals belong to amphibian and reptile category, respectively. Both the animals are known for their cold blood.