  We present some general results on properties of the bihamiltonian
cohomologies associated to bihamiltonian structures of hydrodynamic type, and
compute the third cohomology for the bihamiltonian structure of the
dispersionless KdV hierarchy. The result of the computation enables us to prove
the existence of bihamiltonian deformations of the dispersionless KdV hierarchy
starting from any of its infinitesimal deformations.
