A．正确
B．错误

A．作用在弹簧两端的力是一对作用力与反作用力
Forces applied at both ends of a spring is a pair of action and reaction force.
B．在进行组装时，需要对弹簧平衡方程的矩阵形式进行扩充，扩充后方程仍是线性方程组
When assembling the expanded spring equilibrium equations in matrix form, after the augmentation, the equations are still linear.
C．在进行组装时，仅需对K矩阵进行对应位置上元素进行叠加
When assembling the stiffness matrix K, one needs only to superimpose the different elements on the corresponding position.

A．  ①、③
B．①、③、④
C． ②、③、④
D．②、④