27. He is above ______ in his lessons. That's the results of his hard work.

A．average

B．all

C．anyone

D．Everyone

设n阶实对称矩阵A的秩为r，且满足A²=A，求：
(1)二次型x^TAx的标准形；
(2)行列式|E+A+A²+…+Aⁿ|的值，其中E为单位矩阵。