根据短文内容，回答下列问题。
   Linda and Mary are good friends. Linda is thirteen. She comes from the U.S.A. Mary is twelve. She's from London(伦敦). They study in the same school, but in different classes. Linda's father teaches English in a middle school. Her mother drives a car. Mary's mother works in an office. Her father is a doctor. Linda can speak English, French(法语) and a little Chinese. Linda's mother likes cooking. She likes eating rice. But Linda' s father doesn't. He likes drinking milk and eating bread. Mary's mother doesn't like cooking. But she likes doing some shopping. She can make nice skirts and dresses. Mary's father likes doing housework(家务).

小明同学做凸透镜成像实验时，将点燃的蜡烛放在凸透镜前30cm处，此时在光屏上得到清晰的倒立、放大的实像，则该凸透镜的焦距可能是（　　）