单元刚度矩阵反映了该单元杆端位移与杆端力之间的关系，均具有对称性。
参考答案：对
I’ll never forget such an attractive city ___ I spent many happy hours there with my classmates last summer holidays.
A．that
B．where
C．in which
D．because
某同学画的表示磁场B、电流I和安培力F的相互关系如图所示，其中正确的是 [ ]
A．
B．
C．
D．
