Meaning: 'Common' means shared by two or more entities, indicating that all the mentioned creatures have a similar feature.

Wrong options:

• lunch: The word 'lunch' refers to a meal and does not fit contextually as it does not relate to a shared characteristic among the animals.
• species: While 'species' refers to a group of organisms, it does not convey the idea of a shared trait among the different animals listed, making it contextually inappropriate here.
• naturally: The word 'naturally' implies something inherent or occurring in nature, but it does not grammatically fit as a noun in this context.

Correct answer: directions

Explanation: The word 'directions' is appropriate as it refers to the orientation or path of the magnetic fields, fitting grammatically as a plural noun that aligns with the context of altering magnetic fields affecting brain activity.

Meaning: 'Directions' means the course along which something moves or points, in this case, the orientation of magnetic fields.

Wrong options:

• dispositions: 'Dispositions' refers to a person's inherent qualities or tendencies, which does not fit the context of magnetic fields and their effects on brain activity.
• penchants: 'Penchants' indicates a strong liking or inclination, which is not relevant to the scientific context of magnetic fields and their directional changes.
• reasons: 'Reasons' implies explanations or justifications, which does not align with the context of altering magnetic fields.

Correct answer: recorded

Explanation: The word 'recorded' is the correct past participle form that fits grammatically in the passive voice construction of the sentence, indicating that the brain activity was captured during the experiment.

Meaning: 'Recorded' means to have captured or documented data, in this case, the brain activity of participants.

Wrong options:

• recording: 'Recording' is a present participle and does not fit grammatically in the past tense context of the sentence.
• records: 'Records' is a noun and does not fit grammatically as a verb in this context.
• record: 'Record' is the base form of the verb and does not fit the past tense structure required here.

Correct answer: while

Explanation: The word 'while' serves as a conjunction that indicates simultaneous actions, fitting grammatically and contextually as it connects the two actions occurring during the experiment.

Meaning: 'While' means during the time that; it indicates that one action is happening at the same time as another.

Wrong options:

• since: 'Since' implies a starting point in time and does not convey the simultaneous nature of the actions described in the sentence.
• thus: 'Thus' indicates a conclusion or result, which does not fit the context of two actions occurring at the same time.
• perhaps: 'Perhaps' suggests uncertainty and does not fit the definitive nature of the actions described.

Correct answer: changes

Explanation: The word 'changes' is the correct verb form that fits grammatically in the present tense, indicating that the vertical angle of the magnetic field is subject to variation in the northern hemisphere.

Meaning: 'Changes' means to undergo a transformation or alteration, in this case, referring to the variation in the magnetic field's angle.

Wrong options:

• improves: 'Improves' suggests a positive change, which does not accurately describe the nature of the magnetic field's angle in this context.
• declines: 'Declines' implies a decrease, which does not fit the context of the magnetic field's angle that can vary in different ways.
• leans: 'Leans' suggests a tilt or inclination, which does not accurately capture the concept of variation in the angle of the magnetic field.