Directions: In the following sentence, four words given in bold are marked as (A), (B), (C), and (D), which are possible pairs to be interchanged. Choose the pair(s) of words that need to be interchanged to make the sentence grammatically correct and meaningful. If no interchange is required, mark option 5 i.e., "No interchange required" as your answer.

When searching for telltale (A) life, scientists seek extraterrestrial (B) signs of alien existence by examining the chemical composition (C) of distant atmospheres, as Earth's own atmosphere retains (D) such traces.