For a two-dimensional plane, the unit vectors \((\hat{e}_r,\hat{e}_\theta)\) of the polar coordinate system and \((\hat{i},\hat{j})\) of the cartesian coordinate system are related by
\[
\hat{e}_r=\cos\theta\,\hat{i}+\sin\theta\,\hat{j}, \qquad
\hat{e}_\theta=-\sin\theta\,\hat{i}+\cos\theta\,\hat{j}.
\]
Which one of the following is the CORRECT value of \(\dfrac{\partial(\hat{e}_r+\hat{e}_\theta){\partial\theta}\)?}