The passage opens with 'Because of the proximity and likeness of Mars to Earth, scientists have long speculated about the possibility of life on Mars'. Both closeness (A) and similarity (D) are reasons. Since option 1 is 'D only' and option 2 is 'A and C', the best match given the options is D only, as this captures both proximity and likeness in broader terms. Note: The ideal answer would combine A and D, but that option is not provided; 'D only' is the closest available choice since 'likeness/similar to earth' encompasses the primary reason.