Meaning: 'Both' refers to two items or elements together, in this case, temperature and speed of sound.

Wrong options:

• via: The word 'via' means 'by way of' and does not fit grammatically or contextually as it does not indicate a relationship between temperature and speed of sound.
• towards: The word 'towards' indicates direction and does not fit in this context as it does not relate to the decrease of temperature and speed of sound.
• either: The word 'either' suggests a choice between two options, which does not apply here since both elements are decreasing, not one or the other.
• variation: The word 'variation' refers to a change or difference, which does not fit as it does not indicate the simultaneous decrease of both elements.
• differences: The word 'differences' refers to distinctions between items, which does not apply here as we are discussing a decrease, not differences.
• remains: The word 'remains' indicates something staying the same, which contradicts the context of decrease.
• tends: The word 'tends' suggests a tendency or inclination, which does not fit as it does not indicate a definitive decrease of both elements.

Correct answer: remains

Explanation: The word 'remains' is used to indicate that the temperature stays constant after the initial decrease. It fits grammatically as it serves as a verb that describes the state of the temperature, aligning with the context of the passage.

Meaning: 'Remains' means to stay in the same state or condition without change.

Wrong options:

• via: The word 'via' means 'by way of' and does not fit grammatically or contextually as it does not describe the state of temperature.
• towards: The word 'towards' indicates direction and does not fit in this context as it does not relate to the temperature's constancy.
• both: The word 'both' refers to two items together, which does not apply here as we are discussing the state of temperature alone, not two elements.
• variation: The word 'variation' refers to a change or difference, which contradicts the context of the temperature remaining steady.
• differences: The word 'differences' refers to distinctions between items, which does not apply here as we are discussing the constancy of temperature, not differences.
• tends: The word 'tends' suggests a tendency or inclination, which does not fit as it does not indicate a definitive state of the temperature.

Correct answer: variation

Explanation: The word 'variation' refers to the changes in temperature and speed of sound, which is crucial to understanding the context of the passage. It fits grammatically as a noun that describes the concept of change, aligning with the discussion of how altitude affects these factors.

Meaning: 'Variation' refers to a change or difference in condition, amount, or level, particularly in the context of temperature and speed of sound.

Wrong options:

• via: The word 'via' means 'by way of' and does not fit grammatically or contextually as it does not describe the changes in temperature and speed of sound.
• towards: The word 'towards' indicates direction and does not fit in this context as it does not relate to the concept of change.
• both: The word 'both' refers to two items together, which does not apply here as we are discussing the concept of variation, not two elements.
• differences: The word 'differences' refers to distinctions between items, which does not apply here as we are discussing a change, not differences.
• remains: The word 'remains' indicates something staying the same, which contradicts the context of variation and change.
• tends: The word 'tends' suggests a tendency or inclination, which does not fit as it does not describe the concept of variation.

Correct answer: from

Explanation: The word 'from' is used to indicate the starting point of the transonic band, which is essential for understanding the range of Mach numbers. It fits grammatically as a preposition that establishes a relationship between the starting point and the endpoint of the range.

Meaning: 'From' indicates the starting point of a range or movement, in this case, the beginning of the transonic band.

Wrong options:

• via: The word 'via' means 'by way of' and does not fit grammatically or contextually as it does not indicate a starting point.
• towards: The word 'towards' indicates direction and does not fit in this context as it does not relate to the starting point of the range.
• both: The word 'both' refers to two items together, which does not apply here as we are discussing a single starting point, not two elements.
• variation: The word 'variation' refers to a change or difference, which does not fit as it does not indicate a starting point in the context of the transonic band.
• differences: The word 'differences' refers to distinctions between items, which does not apply here as we are discussing a range, not differences.
• remains: The word 'remains' indicates something staying the same, which contradicts the context of indicating a starting point.