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.

Which of the following contributed most directly to the trend shown above?
A. advances in medical technology
B. changes in immigration policy
C. No Child Left Behind legislation
D. Reductions in Cold War tensions