The author suggests that the realization described in the final sentence of the passage had which of the following effects? A. It introduced a new topic for study by astronomers interested in planetary formation. B. It led astronomers to adopt a number of different strategies in trying to determine the composition of cometary interiors. C. It called into question an assumption that astronomers had made about comets. D. It cast doubt on astronomers' ability to study the interior parts of comets. E. It caused astronomers to revise their account of the composition of the outer planets.