Meaning: 'Defined' means to explain the meaning of something clearly and precisely, indicating how a force is understood in physics.

Wrong options:

• stated: 'Stated' implies a declaration but does not convey the essence of characterization needed here.
• expressed: 'Expressed' suggests communication rather than a formal definition, making it less suitable in this context.
• measured: 'Measured' refers to quantifying something, which does not apply to the conceptual definition of a force.

Correct answer: that

Explanation: The word 'that' is a relative pronoun used to introduce a defining clause, correctly linking gravity to the forces it governs. This is grammatically and contextually appropriate as it specifies which forces are being discussed.

Meaning: 'That' is used to introduce a clause that provides essential information about the noun it follows, in this case, the forces governing the universe.

Wrong options:

• who: 'Who' is used for people, not applicable here as it refers to forces, which are not human.
• by which: 'By which' is more formal and implies a method rather than a defining relationship, making it less suitable.
• whom: 'Whom' is also used for people and is incorrect in this context.

Correct answer: dictate

Explanation: The word 'dictate' fits grammatically and contextually as it conveys the idea of controlling or determining how interactions occur, which aligns with the overall meaning of the sentence about the forces of physics.

Meaning: 'Dictate' means to control or determine something, indicating how the forces influence interactions in the universe.

Wrong options:

• discuss: 'Discuss' implies conversation or examination, which does not fit the context of how forces influence interactions.
• order: 'Order' suggests arrangement rather than control, making it less relevant in this context.
• pronounce: 'Pronounce' means to declare or state, which does not convey the intended meaning of influence or control.

Correct answer: have known

Explanation: The phrase 'have known' is in the present perfect tense, indicating an action that started in the past and continues to the present, which is appropriate for discussing knowledge accumulated over time.

Meaning: 'Have known' means to possess knowledge about something from the past until now, indicating a long-standing understanding of gravity.

Wrong options:

• knew: 'Knew' is the simple past tense, which does not convey the ongoing nature of the knowledge about gravity.
• know: 'Know' is present tense, which does not fit the context of discussing historical knowledge.
• are knowing: 'Are knowing' is not a standard form in English and does not fit grammatically.

Correct answer: physicists

Explanation: The word 'physicists' is the correct term as it refers to scientists who study physics, which is relevant to the context of gravity and fundamental forces. This fits both grammatically and contextually.

Meaning: 'Physicists' are scientists who specialize in the study of matter, energy, and the fundamental forces of nature.

Wrong options:

• psychologists: 'Psychologists' study behavior and mental processes, which is unrelated to the study of physical forces.
• physiologists: 'Physiologists' focus on biological functions, not the fundamental forces of physics, making it irrelevant here.
• psychiatrists: 'Psychiatrists' are medical doctors specializing in mental health, which does not relate to the context of physics.