Are the following data time series? If not, explain why.
a) Reports from the Bureau of Labor Statistics on the number of U.S. adults who are employed full- time in each major sector of the economy.
b) The quarterly Gross Domestic Product (GDP) of France from 1980 to the present.
c) The dates on which a particular employee was absent from work due to illness over the past two years.
d) The number of cases of flu reported by the CDC each week during a flu season.