题目

       Cancer is a terrible disease. Every year in the UK, more than 330,000 people get cancer, and doctors usually use a kind of therapy called radiotherapy to treat people. This treatment uses high energy X-rays to destroy the disease, but the effects of treatment can be nearly as bad as the cancer. As the X-rays destroy the cancer cells, they also damage healthy cells that are next to them. It’s like using a shotgun to kill an insect. When really important parts of a person’s body are hurt by the X-ray energy, it can have very bad results. It can also make more cancer grow下列运算正确的是（　　）A．a3-a2=aB．a3?a2=a5C．a3+a=a4D．（a2）3=a5