科研人员最近发现放射性同位素钬Ho可有效地治疗肝癌．该同位素原子核内中子数与质子数之差为（　　）

    On Sunday, Lucy and I go to a bookshop, then we want to go to the supermarket. But we don't know the way how to get to the bookshop. We ask the policeman in the street for help. He tells us to walk along Renmin Road, and turn right at the second crossing（十字路）. Then we walk and walk but we don't see the bookshop. We are too tired so we take a taxi. “Please take us to the bookshop.” We tell the taxi driver. The taxi driver laughs and says, “ The bookshop is over there, you may get off now.”