In each of the following questions, a sentence is given with two blanks, indicating that parts of the sentence are missing. Choose the most suitable pair of words from the given options that accurately and grammatically complete the sentence. Decades of radio silence had led astrophysicists to assume the distant galaxy was chemically ________, but the sudden detection of complex organic molecules suggested a surprisingly ________ environment.

The conjunction "but" indicates a contrast between the past assumption and the new discovery. Because of "radio silence," scientists assumed the galaxy was dead or inactive, making "inert" (chemically inactive/deficient in active properties) ideal for the first blank. The discovery of complex organic molecules contradicts this, showing the environment is actually rich, fertile, or capable of life, which perfectly matches "fecund" (highly fertile or productive).  o (a) fails because if they assumed it was "dynamic" (active), the discovery of molecules would not cause a logical contrast. o (b) is correct. o (c) fails because "redundant" (unnecessary) makes no contextual sense regarding chemical properties. o (d) fails because "sterile" means barren, which contradicts the detection of life-supporting organic molecules.