A Statistical Technique Called Individual Fixed Effects Compares an Outcome

A statistical technique called individual fixed effects compares an outcome across time as some factor changes.All of the following are examples of this technique EXCEPT analyzing:

A) whether having children makes people happier by asking couples before they have their first child and then again afterward.
B) whether people who exercise three times a week have more energy by asking individuals before they start their exercise regimes and then again after they've been exercising for a set period of time.
C) the effects of gender by comparing people's experiences before undergoing sex changes and then again after their sex changes (e.g., like the Donald to Deirdre case mentioned in your text) .
D) whether vegetarians are less depressed than nonvegetarians by comparing the depression levels of vegetarians and nonvegetarians.