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.