题目内容：

 

You're not the same person you were as a child. You're not even the same personyou were five years ago. We don't just mean your personality. While we'd love tostart a never-ending discussion over the “true self”，we're here to talk about thecells（细胞）in your body.

Do you know how your fingernails and hair are constantly growing and replacingthemselves? The same thing happens almost everywhere in your body. Your outerlayer of skin makes way for fresher skin, and inside your veins（血管），blood cells turn over to make wayfor fresh blood. It's probably no surprise that these types of cells are short-lived, even some of the mostseemingly permanent（永久的）parts of your body replace themselves over time-including your bones.

Strong as your bones might seem, the cells they're made of won't last a lifetime. Bones are masters at rebuilding themselves, which is why most broken bones can heal on their own into the right shape.

Bones have special cells called osteoclasts（破骨细胞）whose job is to break down bone, even if itisn't broken, and send the minerals into the blood stream. This may sound frighteningly like your bonesare wearing away, but your body knows what it's doing. As osteoclasts break down bone, osteoblasts（成骨细胞）build up new bone to replace the old. By the end of each year, about 10 percent of yourentire adult skeleton will have been rebuilt through this process.

If 10 percent of your bone breaks down every year, it must take about ten years to renew 100percent of your skeleton. That would be a great guess, and it lines up nicely with the medical myth thatthe human body's cells are fully replaced every seven years, but the body is a bit more complex. That10 percent is just an average-some bone grows faster than that, and some grows slower. While someparts of your bone will turn over quickly, other parts will stick around for decades. In fact, most bonecells could live as long as 50 years.

So before you get wrapped up in an identity crisis（危机）about the cells in your body disappearingon you, know that you can lose some hair and you can cut your nails, but the bones of you aren't goinganywhere-at least not very soon and not all at once.

1.Which of the following is TRUE according to the passage?

A. All the cells in our body are the same ones that we were born with.

B. Our blood cells are constantly growing and replacing themselves.

C. It must take about 10 years to rebuild 100 percent of our skeleton.

D. Osteoblasts can break down skin and also rebuild it over time.

2.The underlined words "wearing away" probably mean“＿＿”．

A. building up B. taking off C. dying out D. turning on

3.What does the passage mainly talk about?

A. Our skeleton seems permanent but renews itself over time.

B. It is not surprising that some cells in our body are short-lived.

C. There are two types of cells in our bones: osteoclasts and osteoblasts.

D. We are not the same person we were five years ago because of personality.