An Increase About 1,000 Years Ago in ¹³C Content of Human

An increase about 1,000 years ago in ¹³C content of human collagen from archeological sites in eastern North America probably records

A) replacement of vegetables and grains in the diet by meat.
B) replacement of corn by beans and squash in the diet.
C) increasing consumption of corn.
D) increasing levels of ¹³C in soils.
E) increasing levels of ¹³C in the atmosphere.