Which of the Following Is an Example of a Time-Series

Which of the following is an example of a time-series data set?

A) amount of labor employed in each factory in the U.S.in 2010
B) amount of labor employed yearly in a specific factory from 1990 through 2010
C) average amount of labor employed at specific times of the day at a specific factory in 2010
D) All of the above are time-series data sets.