Match the autoimmune diseases in Group I with the corresponding primarily affected organ in Group II.
\[\begin{array}{c c} \textbf{Group I} & \textbf{Group II} \\ \text{P. Hashimoto's disease} & \text{1. Brain} \\ \text{Q. Juvenile diabetes} & \text{2. Pancreas} \\ \text{R. Multiple sclerosis} & \text{3. Skeletal muscle} \\ \text{S. Myasthenia gravis} & \text{4. Thyroid} \\ \end{array}\]