  We give two proofs of a conjecture of Hori and Vafa which expresses the
J-function (i.e, the generating function for 1-point descendant Gromov-Witten
invariants) of a Grassmannian in terms of the J-function of a product of
projective spaces. Similar relations are obtained for two-point descendants,
and three-point (primary) Gromov-Witten invariants. As an application we prove
Givental's "R-Conjecture" - hence the Virasoro conjecture - for Grassmannians.
