单选题 《复合题被拆开情况》 Scientists have spent 14 years scanning the brain of a yoga guru (大师). Their findings suggest daily meditation could help protect the brain from aging. According to a study published in the journal Neurocase, Mahinder, who is 41 years old, has a brain almost a decade younger than his actual age. The study’s authors suggest decades of regular meditation could explain the difference. Researchers scanned Mahinder’s brain four times over 14 years. The results were compared with scans from 105 adults aged 25 to 66, taken between 2010 and 2011, using artificial intelligence taught to estimate a person’s age based on the structure of their brain. While the brain age of the control group aligned closely to their calendar age, that of Mahinder did not. The AI system guessed his age was 33—eight years younger than his calendar age. The authors hypothesize Mahinder’s comparatively young brain age could be the result of decades of meditation. This is backed up by previous studies that suggest meditation could slow the wearing down of the shield-like structures that protect the tips of chromosomes (染色体) from deterioration, and the loss of gray matter, both of which contribute to aging. However, the study published in Neurocase is based on observations and correlations, and does not confirm a cause and effect. There may be several other factors (such as genetics) entirely unrelated to meditation that could be a factor here. To complicate the matter further, the study suggests the effect of meditation may not be as simple as slowing the aging process because Mahinder’s brain also appeared to age faster than the control group. The authors note limitations with their study and point out the framework of the experiment is not equipped to confirm whether or not there is a direct relationship between Mahinder’s brain age and meditation practice. Furthermore, Mahinder’s brain scans were compared to those of Americans, who are likely to have a very different lifestyle and come from a very different environment to that of a yoga guru.《问题》：What have previous studies found?