题目

Facial recognition is becoming increasingly common in China, where it has been installed at ATM machines and KFC restaurants. Now the technology has arrived in university classrooms to track student attendance.Shen Hao, a professor with Communication University of China, is using facial recognition in his six courses to keep track of the attendance of more than 300 students.Before every class, Shen activates a new attendance system installed on his tablet computer. Students stand in front of the screen one by one to have their photos taken and matched with those in the school’s database.The 已知方程x2+bx+a=0有一个根是-a（a≠0），则下列代数式的值恒为常数的是（ ）A．abB．C．a+bD．a-b