Why are these groups specifically distinguished? What does this question mean?

Amy gained cardiovascular health benefits from running consistently over the past year. now she runs inconsistently and has lost some of those health benefits. this principle is known as?

According to the article, when our sleep/wake cycle breaks down, we are at an increased risk for all but which select one: a. respiratory illnesses b. dementia c. diabetes d. heart disease

[120 total points] (public health) in one of the largest human experiments ever conducted, in 1954 a randomized controlled trial was run to see whether a vaccine developed by a doctor named jonas salk was effective in preventing paralytic polio. a total of 401,974 children (ages 6–9), chosen to be representative of those who might be susceptible to the disease, were randomized to two groups: 200,745 children (the control group c) were injected with a harmless saline solution (a placebo) and the other 201,229 children (the treatment group t) were injected with salk’s vaccine.what was the point of giving saline solution to the children who didn’t get the vaccine? explain briefly.