Since these years, American adults have improved the most in ______.

A．Plants that absorb metals are being studied by scientists.
B．Scientists are trying to create cancer-fighting crops.
C．Some wild plants can store micronutrients naturally.
D．Anti-cancer products in the foods are good to people's diet.

A．they don't absorb enough micronutrients from their diet
B．they don't have plants that take up metal in their tissues
C．there is no locoweed in their countries
D．there is no iron and zinc containing in their food