Study of Semileptonic Decays $D \to K/\pi e^+ \nu_e$ at CLEO-c

Using data collected at the $\psi(3770)$ resonance with the CLEO-c detector, we measure absolute branching fractions as a function of $q^2$, the invariant mass of the $e^+ \nu_e$ pair, for $D^0 \to K^- e^+ \nu_e$, $D^0 \to \pi^- e^+ \nu_e$, $D^+\to K_S^0 e^+ \nu_e $ and $D^+ \to \pi^0 e^+ \nu_e$. Our measurements of branching fractions, and most of our form factor measurements are the most precise to date. Using unquenched Lattice QCD calculations of the form factor magnitudes we extract the CKM matrix elements $|V_{\rm cs}|$ and $|V_{\rm cd}|$. Our measurement of $|V_{\rm cs}|$ is the most precise direct determination to date.