If you're like most kids, you probably think you'll never get old. Achy joints, failing eyesight, heart attacks: These are things you won't have to deal with for a long time, right？ So why worry now？
As it turns out, the choices you make now can make a big difference in how you feel later in life. I recently learned this lesson the hard way.
It started with an injury: a cracked shinbone caused by too much running on hard pavement. My doctor suggested a bone scan, which showed that my bones axe weaker than average. I don't have osteoporosis, a disease that causes older people to shrink in height and break bones easily. But I'm close.
For me, the diagnosis was a scary wakeup call. I'm just 27 years old, but already I'm worried about things that normally happen only to women more than twice my age. Will I break my hip if I slip on a patch of ice？ Is it sale for me to go skiing, lift heavy boxes, play Ultimate Frisbee？
Perhaps what upsets me most is the realization that I might have avoided all of this if only I had thought ahead earlier in life. Childhood and adolescence are the most important times to build strong bones. For you, there's still time. Doctors suggest a variety of foods you can eat and exercises you can do as a teenager to build strong bones for life.
Getting the right kind of bone-building nutrition and exercise as a teenager is like putting money in the hank. Your bones can stay strong, as you get older.
Unfortunately, many teenagers don't think about their bones when they order lunch or decide what to do with their free time. They'd rather snack on chips or slurp soda than think about vitamins.
And parents don't always set the best example. "I was standing by the elevator at Children's Hospital," says Susan Coupey, an adolescent medical specialist at Children's Hospital at Montefiore in Bronx, New York. "There was a 2-year-old child being fed soda by his parents."
The writer's own experience told readers that ______.

A. they begin to shrink in height and break bones easily as they grow older
B. they can avoid the writer's troubles if they think over them later in life
C. they should ask their parents to set the best examples for them
D. the choices they make now can make a big difference in how they feel later in life