根据以下材料，回答54-57题
The banana“tree”is actually not a real tree．This is because there is no wood in the stem(树干) rising above the ground．The stem is made up of leaves growing very close together,one inside the other．The leaves spread out at the top of the stem and rise in the air．
Banana plants need a lot of care and attention．They must be provided with water if the normal rainfall doesn´t supply enough．The area around the plants must be kept free of weeds(杂草)and grass．
About nine or ten months after planting,a flower appears on the banana plant．This flower is at the end of a long stalk(茎),which grows from the base up through the center of the stem and turns downward when it comes out from the top．Small bananas form on this flower stalk as it grows down-ward．Bananas really grow upside down．As the small bananas form on the stalk,they point down-ward,but as they grow they turn and point upward．
Bananas are harvested while they are still green．Even when they are to be eaten where they are grown,they are not allowed to ripen on the plant．A banana that turns yellow on the plant loses its taste．
The first paragraph in the text mainly discusses ．

A why the stem of the tree is wood

B how the banana grows on the stem

C why the banana tree is not a tree

D how the leaves grow out of the stem