Carol and Susan are very good friends. They are in the same class at school and they often go to each other's home on Sunday. Now they are both eight yeas old. Carol's mother has got a new baby. Carol is very glad to have a little sister, so she is always talking about her to Susan. At first Susan is very interested   in the new baby because she does not have any brothers or sisters. But after some time she begins to get tired of   Carol's keeping on talking about it. She also feels a little jealous   of her friend. One morning when the two girls meet at the school gate, Carol says to Susan cheerfully  , "Do you know, Susan, my baby sister has put on about half a pound in weight   this week?" "That is not very much," answers Carol. "I know a baby that puts on ten pounds a day." "Oh, that can't be true!" answers Carol. "Whose baby is it?" "An elephant's," says Susan. Why do Carol and Susan often go to each other's home? Because   _  .
they are in the same class they are good friends they like to talk about babies they are in the same study group