15. A uniform electric field of 200 V/m is directed at 30° below the x axis, as shown in fig. 1. Find the changes in potential: Vb - Va. The coordinates are b = (2 cm, 3 cm) and a = (-2 cm, 0 cm).
(A) -3.9 V,
(B) 3.9 V,
(C) -9.9 V,
(D) 9.9 V.

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