The graph shows changes in life expectancy in the United States over a fifty-year period. A line graph titled Life Expectancy in the United States shows years on the x axis and age on the y axis. The line starts at 70 in 1960, to 71 in 1970, 74 in 1980, 75 in 1990, 76 in 2000, to 78 in 2010.

What challenge arose as a result of the trend shown above?
A. educating millions of school-age children
B. providing medical care for an aging population
C. keeping people from leaving the country to retire overseas
D. offering easy-to-follow voting registration procedures