Meaning: 'Are formed' means that stromatolites are created or developed through a process involving layers of cyanobacteria and sediment.

Wrong options:

• has been formed: This option is in the present perfect passive tense, which suggests a completed action rather than an ongoing process, making it unsuitable here.
• will form: This future tense implies that the formation will happen later, which does not fit the context of discussing existing fossils.
• were forming: This past continuous tense suggests an action that was ongoing in the past, which does not accurately describe the current state of the fossils.

Correct answer: date

Explanation: The verb 'date' is used here to indicate the time period to which the fossils belong. It fits grammatically as it is in the present tense, aligning with the context of discussing the age of the fossils.

Meaning: 'Date' means to determine the time period or age of something, in this case, the fossils being traced back to a specific time.

Wrong options:

• trace: While 'trace' could imply following back to a time, it does not fit grammatically as it would require a different structure, such as 'traced back.'
• return: This verb suggests coming back to a previous state, which does not apply to the context of determining age.
• reach: This option implies arriving at a point, which does not convey the same meaning as establishing the age of the fossils.

Correct answer: dramatically

Explanation: The adverb 'dramatically' effectively conveys a significant increase in oxygen levels, fitting the context of the sentence which discusses a crucial period in Earth's history. It aligns with the tone and meaning of the overall text.

Meaning: 'Dramatically' means in a striking or significant manner, indicating that the increase in oxygen levels was substantial and noteworthy.

Wrong options:

• commonly: This adverb suggests frequency rather than intensity, which does not capture the significant nature of the increase in oxygen levels.
• smoothly: This implies a gradual or easy process, which contradicts the idea of a significant change during the Great Oxygenation Event.
• thoroughly: This suggests completeness but does not convey the intensity or significance of the increase in oxygen levels.

Correct answer: capable

Explanation: The adjective 'capable' indicates that the stromatolites had the ability to perform oxygenic photosynthesis, fitting grammatically and contextually within the sentence. It describes the potential of the stromatolites based on the evidence found.

Meaning: 'Capable' means having the ability or power to do something, in this case, to perform oxygenic photosynthesis.

Wrong options:

• able: While 'able' is similar in meaning, it is less formal and does not fit as well in the scientific context of the sentence.
• likely: This suggests a probability rather than ability, which does not accurately describe the stromatolites' function.
• suitable: This implies appropriateness rather than ability, which does not convey the intended meaning of having the capacity to perform photosynthesis.