题目

A healthy dose of sunshine may be the secret to staying young, British scientists have disclosed. Vitamin D is produced naturally by the skin in response to sunlight and may help to slow the ageing process and protect against heart disease, according to the study. Researchers from King's College London studied 2,160 women aged between 18 and 79, looking at their telomeres - a biological marker of ageing found in DNA．As people get older, their telomeres get shorter and they are easy to have illnesses. But the study found women with high levels of vitamin D had comparatively longer telomeres -已知点P为线段AB的黄金分割点（AP>BP）,且AB=2，求BP的长．