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 questions from 15 to 19.

Florists, personal trainers, professors, and nutritionists all have healthy jobs according to research. There are many reasons for this. A florist, for example, sells flowers and plants to customers. This is very relaxing work. A personal trainer, on the other hand, is usually very fit. He or she trains other people to lose weight and to keep fit.

However, research also shows that some jobs are 'unhealthy'. They cause a lot of stress and worry. For example, a taxi driver drives all day. He often faces traffic jams and difficult passengers too. A firefighter puts out dangerous fires and sometimes saves people from burning buildings.

Most people are looking for the perfect job.They want a job that is interesting, with a good salary and not much stress. However, it is difficult to get everything. Healthy, relaxing jobs are usually not very well paid. On the other hand, people with stressful jobs often get good salaries. For example, the head of a big company gets a lot of money, but he or she does not always have good health, or the time to enjoy life.

The word 'They' in paragraph 3 refers to______.