OE

The word “but” signals a contrast from what came before it. The experts had to analyze the recording carefully to know what was being said, so at first the recording must have been “unclear” or “hard to make out.” (D) unintelligible perfectly describes a recorded conversation that is hard to make out, and is therefore the correct answer. Neither (A) disturbing nor (E) esoteric, which means “understood only by the specially initiated,” is a fitting answer. Although the recording may indeed have been disturbing or may have used specialized terminology only understood by a small number of listeners, neither of these words describe a situation in which experts would have trouble determining what was said in the recording. (B) streamlined means “aerodynamic,” and while the plane itself certainly would be streamlined, here the word does not apply to the cockpit recording. Finally, (C) coherent, or “understandable,” is the opposite of what is needed.