请解析这个题目---**Question 7** **Question Stem:** There is a spherical cavity of radius R in a planet of radius 2R, as shown. The distance between center of two spheres d_OO' = R. The planet has a uniform mass density ρ (Except the cavity). The gravitational field at point P, which is a distance R to point O, is: **Options:** A. \frac{\rho\pi GR}{4} B. \frac{\rho\pi GR}{2} C. \rho\pi GR D. 2\rho\pi GR E. 4\rho\pi GR **Chart/Diagram Description:** * **Type:** Cross-sectional diagram showing a larger circle with a smaller circular cavity inside. * **Main Elements:** * **Large Circle:** Represents the planet. Its radius is labeled as 2R, indicated by an arrow extending from the center O to the outer edge. * **Small Circle:** Represents the spherical cavity. Its radius is labeled as R, indicated by an arrow extending from the center O' to the edge of the small circle. * **Centers:** O is the center of the large circle (planet). O' is the center of the small circle (cavity). * **Distance between Centers:** The distance between O and O' is labeled as R, indicated by an arrow between O' and O. * **Point P:** A point located along the line passing through O and O'. P is to the right of O, and the distance from O to P is labeled as R, indicated by an arrow between O and P. * **Shaded Region:** The area between the outer circle and the inner circle is shaded, representing the planet's material with uniform mass density ρ. The density ρ is labeled within the shaded region. * **Labels:** O, O', P, R, 2R, ρ. * **Axis:** A horizontal line passes through O', O, and P, with distances R indicated between O' and O, and between O and P. An arrow points from O to P, indicating a distance of R. Another arrow points from O' to the edge of the cavity, indicating radius R. An arrow points from O to the outer edge of the planet, indicating radius 2R. * **Relative Position:** O' is to the left of O. P is to the right of O. O', O, and P are collinear. The distance O'O is R, and the distance OP is R. * **Annotations:** There is some handwritten text on the image, including "d_OO'", "ρ", "m", "m'", "P", "O", "O'", "R", "R", "2R", and some calculations like "m(-R) + 7m(x) = 0 / 8m" and "R/7". These handwritten annotations appear to be notes or scratch work, not part of the original problem statement or diagram labels.