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1. The gravitational pull of the planets is a fundamental aspect of our understanding of the universe.
行星的引力是理解宇宙的基本方面。
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2. Gravitational waves have been detected by advanced astronomical instruments, providing evidence for the existence of black holes.
高级天文仪器已经探测到引力波，为黑洞的存在提供了证据。
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3. Gravity is a force that acts between two objects, causing them to move towards each other or away from each other.
引力是两个物体之间的力，导致它们相互吸引或远离。