The free-radical theory states that aging is caused by

A) damage to DNA.
B) the body's lowered resistance to stress and disease.
C) biological constraints on cell division.
D) nutritional deficiencies.

The free-radical theory proposes that aging is caused by the accumulation of damage to DNA and other molecules due to reactive oxygen species or free radicals generated during normal metabolism. This damage leads to cellular dysfunction and eventually to the decline in tissue and organ function associated with aging.

Note: Option B is related to the wear-and-tear theory, which proposes that the cumulative effects of stress, disease, and damage to tissues over time are responsible for aging. Option C refers to the limited number of times that a cell can divide before becoming senescent, which is associated with the telomere theory. Option D is a general statement regarding the significance of nutrition for overall health but is not directly linked to any specific aging theory.