In a specific positional encoding method, pairs of embedding dimensions are rotated using a 2x2 matrix defined as: $R_{t\theta_k} = \begin{bmatrix} \cos t\theta_k & \sin t\theta_k -\sin t\theta_k & \cos t\theta_k \end{bmatrix}$ where t is the token's position and \theta_k is a frequency. Given a token at position t = 2 and a frequency \theta_k = \pi/4, which matrix correctly represents the rotation?