Meaning: 'Invisible' means something that cannot be seen, which accurately describes gravity as it is not a visible entity but still exerts a force.

Wrong options:

• unknown: The word 'unknown' implies something that is not known or identified, which does not fit the context of describing a force that is well-established in physics.
• unbelievable: 'Unbelievable' suggests something that is hard to accept as true, which does not apply to the scientific understanding of gravity as a fundamental force.
• inconsistent: 'Inconsistent' means lacking harmony or regularity, which does not accurately describe gravity, as it is a consistent force acting between objects.

Correct answer: theory

Explanation: The word 'theory' is the correct choice as it refers to a well-substantiated explanation of some aspect of the natural world, which aligns with the context of Newton's work on motion. It is a noun that fits grammatically in the sentence.

Meaning: 'Theory' refers to a systematic explanation of phenomena, particularly in science, which in this case pertains to Newton's laws of motion.

Wrong options:

• concept: While 'concept' refers to an abstract idea, it is less specific than 'theory' and does not convey the established scientific framework that Newton provided.
• method: 'Method' implies a systematic procedure or technique, which does not accurately capture the essence of Newton's comprehensive explanation of motion.
• argument: 'Argument' suggests a reasoned discourse or debate, which is not the appropriate term for the scientific explanation provided by Newton.

Correct answer: explained

Explanation: The verb 'explained' fits grammatically as it indicates that Newton clarified or made the concept of gravity understandable, aligning with the past tense used in the sentence.

Meaning: 'Explained' means to make something clear or easy to understand, which is what Newton did regarding gravity.

Wrong options:

• undermined: 'Undermined' means to weaken or damage, which is contrary to the context of Newton's work, as he strengthened the understanding of gravity.
• overturned: 'Overturned' implies reversing or invalidating a previous understanding, which does not apply here since Newton's work built upon existing knowledge rather than negating it.
• realized: 'Realized' suggests coming to an understanding or awareness, but does not convey the act of clarifying or elucidating a concept as 'explained' does.

Correct answer: affect

Explanation: The verb 'affect' is appropriate as it indicates that the Earth and the Moon influence each other through gravitational pull, fitting grammatically and contextually in the sentence.

Meaning: 'Affect' means to have an impact on or to influence, which accurately describes the gravitational interaction between the Earth and the Moon.

Wrong options:

• spin: 'Spin' refers to rotating or turning around an axis, which does not convey the idea of influence or interaction in the context of gravity.
• evade: 'Evade' means to escape or avoid, which is not relevant to the relationship between the Earth and the Moon regarding gravitational pull.
• span: 'Span' means to extend across or cover, which does not fit the context of gravitational influence between the two celestial bodies.