Meaning: 'Where' is used to ask about or specify a location.

Wrong options:

• levels: The term 'levels' refers to stages or degrees, which does not relate to the inquiry about location.
• what: The word 'what' is an interrogative pronoun that asks for information but does not specify a location, making it unsuitable here.
• bases: 'Bases' is a plural noun that refers to foundational elements, which does not fit the context of asking about a location.

Correct answer: using

Explanation: The word 'using' is a gerund that indicates the action of employing a method or system, which is appropriate in the context of explaining how absolute location is determined. It fits grammatically as it follows the preposition 'by,' indicating the means of explanation.

Meaning: 'Using' refers to the act of employing something for a purpose.

Wrong options:

• levels: 'Levels' does not fit as it is a noun and does not convey the action of employing a system.
• what: 'What' is an interrogative pronoun that does not indicate an action and is therefore inappropriate in this context.
• bases: 'Bases' is a noun that does not convey the action of employing a method.

Correct answer: poles

Explanation: The term 'poles' refers to the North and South Poles, which are the endpoints of the Earth's axis. This fits contextually as it describes the extreme points of latitude, and grammatically, it serves as a noun that completes the thought about latitude's range.

Meaning: 'Poles' refers to the two ends of the Earth's axis, specifically the North and South Poles.

Wrong options:

• levels: 'Levels' does not relate to the geographical context of latitude and is therefore incorrect.
• what: 'What' does not fit as it does not refer to a specific geographical location.
• bases: 'Bases' is a noun that does not relate to the concept of geographical extremes.

Correct answer: measures

Explanation: The word 'measures' is a verb that indicates the action of determining the extent or amount of something, which is appropriate in the context of longitude. It fits grammatically as it serves as the main verb in the clause, explaining what longitude does.

Meaning: 'Measures' means to determine the size, amount, or degree of something.

Wrong options:

• levels: 'Levels' is a noun that does not convey the action of determining and is therefore incorrect.
• what: 'What' does not fit as it does not indicate an action related to longitude.
• bases: 'Bases' is a noun that does not relate to the action of measuring.

Correct answer: which

Explanation: The word 'which' is a relative pronoun that introduces a clause providing additional information about the prime meridian. It fits grammatically as it connects the clause to the noun it describes, maintaining clarity in the sentence.

Meaning: 'Which' is used to introduce a clause that provides more information about a noun.

Wrong options:

• levels: 'Levels' does not function as a pronoun and does not provide additional information about the prime meridian.
• what: 'What' is an interrogative pronoun that does not fit the context of providing information about a noun.
• bases: 'Bases' is a noun that does not serve as a connector or descriptor in this context.