Dear Editor,(编辑)
   I am unhappy these days. I am a student in Grade 3 of a middle school. I want to be a policewoman in the future. (未来) But my mother doesn't like  the job. She always says it is too dangerous for a girl to work as a policewoman. She thinks girls should work in the office. She hopes that I can work as a doctor. But I really dislike being a doctor. It's terrible for me to work in the hospital.  When I talk with her about it, she is often angry. She always thinks she is right. I don't know what to do. Can you help me?
                                                                                                                                                         Yours,
                                                                                                                                                         Nancy

下列几种变异中，不能遗传的变异是（   ）

My sister and I look different,           we are twins.