Classify the following semileptonic decays of the D⁺(1869) = cd̅ meson as Cabibbo-allowed, Cabibbo-suppressed or forbidden in lowest-order weak interactions, by finding selection rules for the changes in strangeness, charm and electric charge in such decays:

(a) D⁺ → K⁻ + π⁺ + e⁺ + ν_e, 

(b) D⁺ → K⁺ + π⁻ + e⁺ + ν_e,

(c) D⁺ → π⁺ + π⁺ + e⁻ + ν̅_e, 

(d) D⁺ → π⁺ + π⁻ + e⁺ + ν_e.