阅读理解
Does stress cause gray hair？Many people believe that tension and stress can cause your hair to lose its color.In fact，it is an_old_wives’_tale that being upset all the time can result in premature graying.Here is a closer look at this situation and why some believe that this old wives’ tale could be true.
As one ages，the hairs on the head also age.In fact，just about everything on the human body begins to show signs of wear and tear after a while.The joints wear and become stiff and skin wears and becomes wrinkled.Hairs on the head grow and eventually die.They are replaced，but in the process something else happens.
When new hairs are replaced，the color or pigment (色素) is also added to them.However，as one ages，this pigment becomes less and less available.At first，hairs will have a limited amount of pigment and they will appear to be grayish in color.In time，they will lose all of their pigment and will be white.Many things are known to cause graying，and genetics is a very powerful factor.If one of your parents turns gray at an early age，there is a good chance that you will too.Some people begin the graying process in high school，while others may be over the age of forty.
Constant worry and tension can have many harmful effects on the human body.It is a known fact that stressful times can cause one to suffer hair loss.It would make sense that if it can cause these problems，perhaps it also may contribute to graying.However，there is no sound evidence to support this.
On the other hand，many medical professionals believe that there may be a link between graying and a constant state of anxiety or tension.Does stress cause gray hair？There is no scientific evidence to support the theory，but it is best to avoid stressful situations.
的解是（ ）.
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