题目

11-year-old Courtney Thompson was so passionate about gymnastics that she would practice five hours a day until her arms were sore and her elbows swollen. She rarely complained about her tough schedule, and for good reason. Courtney was ranked the best gymnast in the state for her age , and she dreamed of going to the Olympics. But on January 12th,2005, while practicing a routine exercise, she felt an unbearable pain in the left elbow. The medical examination then revealed a severe injury, resulting in an immediate surgery and a slow, painful recovery.        What happened to Courtney T已知向量，b=(x－4，2，x)，若a⊥b，则实数x的值是 [　　] A．－3，2 B．－4，1 C．－1，4 D．－2，3