5. Following the science fiction, earth is a hollow shell, and there is a cavity in the center of the earth as the inner world. According to gravitation theory, what is the gravity field in that inner world?
(A) same as on the surface of the earth
(B) linear function as the distance from the center
(C) very small
(D) proportional to 1/r²
(E) proportional to 1/r

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