设A为3阶矩阵，
为 A 的属于特征值1的线性无关的特征向量，
为A的属于-1的特征向量，则
的可逆矩阵P为（）
A、
B、
C、
D、
B、
C、
D、
正确答案：
D
答案解析：
Comrade Li ________ be in Beijing because I saw him in town only a few minutes ago.
When he suddenly ________ up the subject of genetic engineering, there was an embarrassedsilence.
In the future, more ______ should be placed on the education of the young.