More women than men suffer from Alzheimer' s disease -- a disease that is most commonly contracted by elderly persons. This discrepancy has often been attributed to women' s longer life span, but this theory may be wrong. A recent study has shown that prescribing estrogen to women after menopause, when estrogen production in the body decreases, may prevent them from developing the disease. Men' s supply of testosterone may help safeguard them against Alzheimer' s disease because much of it is converted by the body to estrogen, and testosterone levels stay relatively stable into old age. Which one of the following most accurately expresses the main conclusion of the argument? ["A decrease in estrogen, rather than longer life span, may explain the higher occurrence of Alzheimer's disease in women relative to men.", "As one gets older, one's chances of developing Alzheimer's disease increase.", "The conversion of testosterone into estrogen may help safeguard men from Alzheimer's disease.", "Testosterone is necessary for preventing Alzheimer's disease in older men."]