37.A parallel-plate capacitor is connected to an ideal battery that maintains a constant voltage V. A dielectric slab initially completely fills the space between the plates. An engineer slowly pulls the slab out at constant speed while the battery remains connected. Which of the following correctly describes the work done by the engineer (Wext) and the change in the electrostatic energy stored in the capacitor (△U)?
(A) Wext is positive and AU is positive.
(B) Wext is positive and △U is negative.
(C) Wext is negative and △U is positive.
(D) Wext =0 and △U is negative.
(E) Wext =0 and△U is positive.

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統計： A(0), B(1), C(0), D(0), E(0) #3847813