My secret of staying young is simple: keeping your mind awake and you will stay young all over. Take an interest in world around you, and learn at least one old thing every day. Don’t think that you are ever very old to go back to school. I know a man who entered medical college in 70. He got his degree with honors and became a famous doctor. Other man went to law school at 71 and is now a active lawyer. You may say that staying young is easy only for those live in the future. In fact, you can do it whether you care enough to try to keep your mind awake and active, and which is the only way to be always young.

由于使用磨损和自然损耗造成的抵押物贬值是（）。

A．功能性贬值

B．实体性贬值

C．经济性贬值

D．泡沫经济