Frogs are amphibians. Like all amphibians, frogs are cold-blooded, meaning that their body temperatures change with the temperature of their surroundings. When temperatures drop, some frogs dig burrows in the mud at the bottom of ponds. They hibernate in these burrows until spring, perfectly still and barely breathing. Frogs can be found just about anywhere there's freshwater, on all continents except Antarctica. Though they thrive in warm, moist tropical climates, frogs also live in deserts and high on 15,000-foot mountain slopes. The Australian water-holding frog is a desert dweller that can wait for seven years for rain. It burrows underground and surrounds itself in a transparent cocoon made of its skin. Frogs' skin is critical to their survival. Through it, they drink and breathe. Frogs don't swallow water; they get the moisture they need through their skin. And although frogs have lungs, they rely on the oxygen they absorb through their skin, especially when they're underwater. Frogs must keep their skin moist. Otherwise, oxygen does not easily pass through it and the frog suffocates. Even though frog skin secretes mucus that helps keep it moist, their skin tends to dry out easily, which is why they usually stay near bodies of water.

Consider the bar graph comparing the number of frogs in four different habitats. Based on the information above about frogs, what is the MOST LIKELY factor limiting the frog population?
A) competition
B) food
C) space
D) water