When a tree is old, the tubes in the centre of the tree don't carry water. The walls of the tubes have become thick with materials which have stuck along them over the years

　　forming a kind of wood called "heartwood". This kind of wood is darker in colour than the young, growing wood on the outside of the tree.

　　You don't very often see whole tree trunks which have been cut across. But once you learn to read a cross section (横截面) of the wood, you can see much more in wood which has been used to make boxes, houses, and other things.

　　In most wood, instead of seeing the trunk cut across, you are seeing it cut along its length. Because you don't see the whole tree, you can't tell how old it was.

　　1. The writer says you can tell the age of a tree by______

　　A. the number of rings in its trunk

　　B. the size of the base of its trunk

　　C. the colour of its rings

　　D. the rings on the outside of the tree

　　2. When the wood of a tree is mostly light in colour, this means that the tree _____.

　　A. grew slowly

　　B. grew quickly

　　C. only grew in winter

　　D. only grew in summer

　　3. Heartwood is wood which is _____.

　　A.older and darker in colour

　　B.fast-growing

　　C.younger and lighter in colour

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