Meaning: 'Stirs' means to move or cause to move slightly, suggesting that in the heart of Antarctica, there is no activity or life.

Wrong options:

• talks: 'Talks' implies communication, which is not relevant in this context as there is no life to communicate.
• suits: 'Suits' suggests appropriateness or fittingness, which does not apply to the context of life in Antarctica.
• sounds: 'Sounds' refers to noise or auditory perception, which is not applicable in describing the absence of life.

Correct answer: that is

Explanation: The phrase 'that is' serves as a clarification or elaboration, fitting seamlessly into the sentence to introduce the exception of Lake Vostok. It maintains the formal tone of the text and logically connects the preceding and following ideas.

Meaning: 'That is' is used to clarify or specify something previously mentioned, indicating that Lake Vostok is an exception to the lack of life.

Wrong options:

• for example: 'For example' suggests a specific instance, which is less formal and does not fit the clarifying role needed here.
• in effect: 'In effect' implies a result or consequence, which does not serve the purpose of clarification in this context.
• as such: 'As such' indicates a consequence or result, which does not align with the need for clarification in this sentence.

Correct answer: spot

Explanation: The word 'spot' is appropriate as it refers to a specific location, fitting the context of describing Lake Vostok as the coldest location on earth. It aligns grammatically and contextually with the preceding description of the lake.

Meaning: 'Spot' means a particular place or location, emphasizing the uniqueness of Lake Vostok's position.

Wrong options:

• tip: 'Tip' suggests the end or peak of something, which does not accurately describe the location of Lake Vostok.
• speck: 'Speck' implies something very small, which is not suitable for describing a significant geographical location like Lake Vostok.
• dot: 'Dot' also implies something small and insignificant, which does not convey the importance of the coldest spot on earth.

Correct answer: probe

Explanation: The verb 'probe' fits well as it means to explore or examine thoroughly, which is contextually relevant to biologists wanting to investigate the depths of Lake Vostok. It aligns with the scientific inquiry theme of the passage.

Meaning: 'Probe' means to investigate or explore deeply, particularly in a scientific context, indicating the desire to uncover secrets of the lake.

Wrong options:

• burrow: 'Burrow' implies digging into the ground, which does not fit the context of exploring a body of water.
• plunge: 'Plunge' suggests a sudden dive, which is less appropriate for the careful investigation implied here.
• tunnel: 'Tunnel' indicates creating a passage, which does not accurately reflect the action of examining the lake.

Correct answer: heels

Explanation: The phrase 'hot on their heels' is an idiomatic expression meaning to follow closely or pursue eagerly. It fits grammatically and contextually, indicating that geologists and glaciologists are actively pursuing their research on Lake Vostok.

Meaning: 'Heels' in this context refers to being closely followed or pursued, emphasizing the urgency and eagerness of the researchers.

Wrong options:

• knees: 'Knees' does not form a recognized idiomatic expression and does not convey the same meaning of pursuit.
• paces: 'Paces' suggests a measure of distance or steps taken, which does not fit the idiomatic context of following closely.
• steps: 'Steps' could imply a process but lacks the idiomatic expression that conveys urgency and pursuit.

Correct answer: witness

Explanation: The verb 'witness' is appropriate as it means to see or experience an event, fitting the context of researchers anticipating the discoveries related to Lake Vostok. It aligns grammatically and contextually with the overall theme of exploration and discovery.

Meaning: 'Witness' means to see or experience something firsthand, indicating that researchers expect to observe significant findings regarding the lake.

Wrong options:

• present: 'Present' implies to show or introduce, which does not fit the context of experiencing discoveries.
• expose: 'Expose' suggests revealing something hidden, which does not align with the anticipation of witnessing discoveries.
• reveal: 'Reveal' implies making something known, which is less about experiencing and more about disclosing information.