$(\cos \alpha+\cos \beta)^{2}+(\sin \alpha+\sin \beta)^{2}$ is
(a) $4 \cos ^{2}\left(\frac{\alpha+\beta}{2}\right)$
(b) $4 \cos ^{2}\left(\frac{\alpha-\beta}{2}\right)$
(c) $-4 \sin ^{2} \frac{\alpha}{2} \cos ^{2} \frac{\beta}{2}$
(d) $4 \sin \frac{\alpha}{2} \cos \frac{\beta}{2}$