Read the following article and select the best answer. Article: Just five minutes of exercise a day outdoors can improve mental health, according to a study. Researchers from the University of Essex, UK have found that as little as five minutes of a "green activity" such as walking, gardening, cycling or farming could help to improve mood and increase self-esteem . Many studies have shown that outdoor exercise can reduce the risk of mental illness and improve a sense of well-being, but the new study, led by Jules Pretty and Jo Barton, was the first to determine how much time needed to be spent on green exercise for the benefits to show. Barton and Pretty looked at data from 1,252 people of different ages, genders and mental health status taken from 10 studies in Britain. They analyzed their different outdoor activities including walking, gardening, cycling, fishing, boating, horse-riding and farming in locations such as a park, garden or nature trail . They found that the mental health changes were seen after just five minutes. With longer periods of time exercising in a green environment, the positive effects were more apparent. Looking at men and women of different ages, the researchers found the health changes were particularly strong in the young and the mentally ill. Green and blue A bigger effect was seen with exercise in an area that also contained water - such as a lake or river. "This is because historically we are drawn to water to survive and it is in our genetic make-up to be at one with nature," the researchers wrote in their report. Pretty said those who were generally inactive, or stressed, or with mental illness would probably benefit the most from "green exercise". "Employers, for example, could encourage staff in stressful workplaces to take a short walk at lunchtime in the nearest park to improve mental health." He also said exercise programs outdoors could benefit youth offenders. Question: What is the main problem discussed in this article? - Physical well-being. - Mental illness. - Outdoor exercise. - Harmony with nature.
A:
Outdoor exercise.