Create a line plot of the average level of health spending across all of the countries in the dataset over time. [Hint: You can use the tapply( ) function to find the average for each year.] Include a line of average/mean health spending for the United States on the plot. [Hints: i) The United States is coded as "United States of America" in the country variable; ii) You may need to adjust the scale of y-axis using ylim in order to see both the average and the United States in one plot.]