A parallel-plate capacitor with vacuum between plates of area \(A\) is initially charged to \(+Q_0\) and \(-Q_0\) on its respective plates at a separation distance \(d_0\), storing an initial electrostatic potential energy \(U_0\). The capacitor is then disconnected from the charging source, leaving it isolated. An external agent slowly pulls the plates apart until their separation distance is \(3d_0\). Assuming edge effects are negligible, which of the following gives the electric field magnitude \(E\) between the plates at the new separation and the work \(W_{\text{ext}}\) done by the external agent during the expansion?