Determine whether each of the following questionable statements is best explained by correlation without causation, an omitted variable, or reverse causation.
a. In cities that have more police, crime rates are higher.
b. Many retired people live in states where everyone uses air conditioning during the summer.
c. More people come down with the flu during the Winter Olympics than during the Summer Olympics.
d. For the last five years, Punxsutawney Phil has seen his shadow on Groundhog Day, and spring has come late.