Ring_of_Polynomial_Forms_is_Integral_Domain

Let $\struct {R, +, \circ}$ be a commutative ring with unity. Let $\struct {D, +, \circ}$ be an integral subdomain of $R$. Let $X \in R$ be transcendental over $D$. Let $D \sqbrk X$ be the ring of polynomials in $X$ over $D$. Then $D \sqbrk X$ is an integral domain.