Which of the following observations is an example of a positive correlation?

When a population consumes more fruits and vegetables, the percentage of people in that population with scurvy decreases.

When a group of 6-year-old children increase their physical activity level to 60 minutes a day, the children’s muscle mass increases.

When older adults increase their daily intake of vitamin D, the percentage of the adults that develops infections decreases.

When teenage girls increase their intake of iron-rich foods, the percentage of the girls who have iron-deficiency decreases.