The stiffness matrix of a bar element is given by

a. \(\frac{E A}{l}\left[\begin{array}{ll}1 & 1 \\ 1 & 1\end{array}\right]\)

b. \(\frac{E A}{l}\left[\begin{array}{rr}1 & -1 \\ -1 & 1\end{array}\right]\)

c. \(\frac{E A}{l}\left[\begin{array}{ll}1 & 0 \\ 0 & 1\end{array}\right]\)