The stabilization of symplectic groups over a polynomial ring

We prove that if $B$ is a polynomial ring over a field, then for $r\geqslant2$, any element of $Sp_{2r}B$ can be written as a product of elementary symplectic matrices over $B$.
We also prove a stabilization theorem for the symplectic $K_1$-functor in the case of polynomial rings and Laurent rings.
Bibliography: 6 titles.