阅读理解：For each blank in the following passage

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For each blank in the following passage there are four words or phrases marked A, B, C and D. Fill in each blank with the word or phrase that best fits the context.

We all know the dangers of a diet that's high in fat. But a new study from researchers at the Tokyo Medical and Dental University (TMDU) in Japan has now found having a fat-heavy diet can also   1  hair loss.

In the average healthy adult, the hair follicle stem cells (HFSCs) (毛囊干细胞)  2  themselves regularly, which is why your hair grows back when you cut it and also why it becomes longer. As with most things in the human body, though, aging doesn't do the HSFCs any   3  . As you get older, the stem cells lose their ability to reproduce as   4   as they once did, leaving the hair to thin and/or fall out.