（）属于特殊矩阵。
A.对角矩阵
B.上三角矩阵
C.下三角矩阵
D.稀疏矩阵
E.对称矩阵
参考答案：A, B, C, E
Only when _________________________.
A．did he arrive the meeting began
B．he arrived the meeting began.
C．did he arrive did the meeting begin.
D．he arrived did the meeting begin.
矿震三相三线变压器中各项负载的不平衡度不许超过（）。
A.O.5
B.O.2
C.0.25
D.0.3
