The correct option to fill the gap in the sentence "Oxygen is necessary to life. Nothing can live without it. It was .... a century ago" is C. discovered. Explanation of the Correct Answer

1. Understanding the Context: The sentence discusses the importance of oxygen to life and implies that there was a time in the past when the significance of oxygen was not fully understood. The phrase "a century ago" indicates that we are referring to a historical event related to the understanding of oxygen.
2. Meaning of "Discovered": The word "discovered" means to find out something that was previously unknown. In the context of oxygen, it refers to the scientific discovery of oxygen as a vital element for life. This aligns perfectly with the historical context of the sentence, as oxygen was indeed discovered in the late 18th century by scientists like Joseph Priestley and Carl Wilhelm Scheele.
3. Grammatical Fit: The structure of the sentence requires a verb that fits the past tense context. "Discovered" is in the past tense and fits grammatically with "It was ... a century ago."

Why the Other Options Are Incorrect

• A. Invented: The term "invented" implies that something was created or made by someone. Oxygen is a natural element that exists in the environment; it was not invented. Therefore, this option does not fit the context of the sentence.
• B. Found out: While "found out" can mean to discover something, it is more informal and less precise than "discovered." It also does not convey the same level of significance as the scientific discovery of oxygen. In formal writing, especially in a scientific context, "discovered" is the more appropriate choice.
• D. Learnt: The word "learnt" (or "learned" in American English) refers to the process of acquiring knowledge or skills. While it could be argued that people "learnt" about oxygen, it does not capture the essence of the scientific discovery that the sentence is referring to. The focus is on the act of discovering oxygen, not merely learning about it.

Summary of Key Points

• The correct answer is C. discovered because it accurately reflects the historical context of oxygen's significance to life.
• "Discovered" fits grammatically and contextually, while the other options do not convey the same meaning or relevance.
• "Invented" suggests creation, "found out" is informal, and "learnt" focuses on knowledge acquisition rather than discovery.
• Understanding the nuances of word choice is crucial in selecting the best option in fill-in-the-blank questions.

By keeping these points in mind, you can better understand how to approach similar questions in the future.