How has the life expectancy for American women changed over the past century or so?
a. It remained stable for the first half of this period, and then rose significantly in the second half
b. It decreased over the first 30 years of the 20th century, and increased over the next 80 years
c. It increased significantly through the 20th century and has remained stable during the 21st
d. It rose significantly over the first 70 years, and then fell somewhat over the next 40 years