The one effect of gravity within the solar system is planet experiences increasing gravity throughout its orbit, so it continuously speeds up, which means letter B.
The force of gravity between two objects is the product of their masses divided by the square of the distance between them. That is, the more mass an object has and the closer it is, the greater its force of attraction on another object.
The concept of gravity is something known naturally, however the gravity that occurs in the solar system and the planets leaves them in orbit and in addition to that with each passing year these planets have an even faster speed.
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