Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the question.

All living cells in an animal's body require energy to power the various chemical processes going in inside them. This energy is ultimately supplied by the food that animals eat. These chemical processes are collectively referred to as metabolism, and one of the byproducts of metabolism is heat. Metabolic rates vary significantly between species. Warm-blooded animals (birds and mammals) have metabolic rates about five to ten times higher than those of similarly sized cold-blooded ones (reptiles, amphibians, and fishes). And it is precisely because birds and mammals have such high metabolic rates that they are able to keep their bodies warm.

The terms warm-blooded and cold-blooded are still in everyday use, but they are not entirely precise. Anyone who has handled a snake knows this because a snake’s body actually feels quite warm. But very little of the snake’s body heat originates internally, from its cells, most of it having been supplied from the outside, either by the sun or by a heat lamp. Instead of referring to reptiles as cold-blooded, they are best described as ectothermic, meaning “outside heat”. Similarly, birds and mammals are said to be endothermic, meaning “inside heat”.

There are advantages and disadvantages to each thermal strategy. Reptiles are usually sluggish first thing in the morning, their body temperatures having dropped during the cool of the night. Accordingly, they have to bask in the sun to raise their body temperatures, but once they have warmed up sufficiently, they can go about their business. By altering between the sun when they are too cool, and the shade when they are too warm, many reptiles are able to maintain their body temperatures at optimum levels of about 95°F or more. Endotherms, on the other hand, maintain temperatures of about 98°F all the time, so they are always ready for action.

I used to keep a crocodile. He had very sharp teeth, and I had to be careful how I handled him during the daytime, when he was warm. But I could do whatever I wanted at night, when he was cold, without any fear of being bitten. The obvious disadvantage of being ectothermic is that the animal’s activity levels are dependent upon the environment. But its low metabolic rates mean that it requires far less food, which is an advantage. I used to feed the crocodile a tiny piece of liver once a week, while the family cat demanded three meals every day. We should therefore not think that reptiles are inferior to mammals and birds; they are just different

What does the word “they” in paragraph 2 refer to?