Question

The early universe contained only the lightest elements, hydrogen and helium. Heavier elements, such as carbon, form only in nuclear reactions in stars and are dispersed when the stars explode. A recently discovered gas cloud contained carbon several billion years ago, when the universe was no more than two billion years old.
If the statements above are true, which of the following must, on the basis of them, also be true?

• A. The earliest stars contained only hydrogen.
• B. Some stars were formed before the universe was two billion years old.
• C. The carbon in the gas cloud later formed part of some stars.
• D. No stars identified to date are as old as the gas cloud.
• E. The gas cloud also contained hydrogen and helium.

Hint:

For logical deduction questions, create a chain of events or a timeline. The premises provide the steps in the chain. The correct conclusion will be a necessary consequence of following that chain of logic.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
This is a logical deduction question. We must accept all the given statements as true and then find the one conclusion that necessarily follows from them. We need to piece together the timeline of events described.
Step 2: Detailed Explanation:
Let's break down the premises into a chronological sequence:
\begin{enumerate} \item The early universe had only hydrogen and helium. \item Heavier elements, like carbon, are created inside stars. \item These heavier elements are dispersed into space only when those stars explode. \item A gas cloud containing carbon was found to exist at a time when the universe was no more than two billion years old. \end{enumerate} Now let's trace the origin of the carbon in that gas cloud. According to the premises, for that carbon to exist, it must have been created inside a star, and that star must have already exploded to release the carbon into the gas cloud.
The entire cycle—star formation, carbon creation within the star, and the star's explosion—must have happened before the gas cloud was observed. Since the cloud was observed when the universe was at most two billion years old, it means that some stars must have formed, lived their lives, and exploded all within the first two billion years of the universe's existence.
Let's evaluate the options based on this deduction:
\begin{itemize} \item (A) The passage states the early universe contained hydrogen and helium. It doesn't restrict the composition of the earliest stars to only hydrogen. \item (B) This is the necessary conclusion. For carbon to exist in a cloud at the two-billion-year mark, stars must have already formed and exploded before that time. This is a direct consequence of the premises. \item (C) The passage doesn't say what happened to the carbon after it was in the cloud. It might have formed new stars, or it might not have. We cannot conclude this. \item (D) The passage gives no information about the age of stars identified "to date." This is outside the scope of the provided statements. \item (E) While it is likely the cloud also contained hydrogen and helium (the most abundant elements), the premises do not force us to conclude this. The focus is solely on the origin of the carbon. \end{itemize} Step 3: Final Answer:
The presence of carbon in a gas cloud at the two-billion-year mark logically requires that some stars had already formed and completed their life cycles by that time.