Measurement of the D Semileptonic Branching Ratio in e+e- Annihilation at the ψ''(3770)

We have observed the ψ'' resonance in the cross section for e⁺e^--->hadrons at E_c.m.=3770+/-6.0 MeV, of total width Γ=24+/-5 MeV and partial width to electron pairs Γ_ee=180+/-60 eV. The cross section for hadronic events which contain anomalous electron provides both unambiguous evidence of D semileptonic decays and a branching ratio measurement of (11+/-2)%.