【单选题】

Skin cancer is the most prevalent of all cancers. The principal cause of skin cancer is overexposure to sunlight, according to most medical experts.Chronic sun exposure-- especially when it causes sunburn or blistering--results in more skin cancer than does any other risk factor, including exposure to x-rays and a family history of the disease. The most effective preventative measure is sun avoidance.

A．exposure to x-rays
B．sun blistering
C．amily history of skin cancer
D．sun avoidance