Read the Paragraph Below and Answer the Questions That Follow

Read the paragraph below and answer the questions that follow.
(1) People who are thinking about getting a pet should carefully consider the kind of care that animal will require. (2) Pets can be classified into three categories according to how labor-intensive they are. (3) The first category includes very labor-intensive pets. (4) These animals require the most time, energy, and money because they need constant, daily care. (5) Horses kept in a barn are probably the best example, for they need to be fed, exercised, and groomed on a daily basis. (6) Dogs, which need to be fed and walked at least once a day, are another good example of a high-maintenance pet. (7) The second category contains those pets that are only moderately labor-intensive, such as a cat or a hamster. (8) They need to be fed every day, but other maintenance is done on only a weekly or monthly basis. (9) For example, a cat's litter box or a hamster's cage needs only periodic cleaning. (10) The third and final category includes very low-maintenance pets. (11) Fish are the best example. (12) They need just a little sprinkling of food every day, and their tanks need cleaning only a few times a year.
On what basis does the writer classify pets?

A) personality
B) labor-intensiveness
C) size