题目

阅读下面的短文，并完成短文后的任务。    Each country has many kind people who volunteer to take care of others．For example （4）some high school and university students in the United States often spend many hours as volunteers in hospitals or old people’s homes． They read books to the people there, or they just visit them and play games with them or listen to their problems．    Other young volunteers go and work in the homes of people who are sick．They clean up or do the shopping．For boys who don't have fathers there is an organization called Big Brothers向空中发射一枚炮弹，经x秒后的高度为y米，且时间与高度的关系为y=ax2+bx+c（a≠0）、若此炮弹在第7秒与第14秒时的高度相等，则在下列时间中炮弹所在高度最高的是（ ）A．第8秒B．第10秒C．第12秒D．第15秒