For each of the following, indicate whether it is a positive linear, negative linear, or nonlinear relationship:
(a) Quality of performance (Y) increases with increased arousal (X) up to an optimal level; then quality of performance decreases with increased arousal.
(b) Overweight people (X) are less healthy (Y).
(c) As number of minutes of exercise increases each week (X), dieting individuals lose more pounds (Y).
(d) The number of bears in an area (Y) decreases as the area becomes increasingly populated by humans (X).