45. A uniformly charged insulating rod is bent into the shape of a semicircle as shown on the right. The rod has a total charge Q and the radius of the semicircle is R. What is the magnitude of the electric field at O, the center of the semicircle? 
(A)
(B)
(C)
(D)
(E)
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統計: A(2), B(2), C(0), D(1), E(0) #3239593
統計: A(2), B(2), C(0), D(1), E(0) #3239593