The file Natural Gas contains the monthly average wellhead and residential price for natural gas (dollars per thousand cubic feet) in the United States from January 1, 2008, to January 1, 2012. For the wellhead price and the residential price:
a. Construct a time-series plot.
b. What pattern, if any, is present in the data?
c. Construct a scatter plot of the wellhead price and the residential price.
d. What conclusion can you reach about the relationship between the wellhead price and the residential price?