Thanks for the question. We're looking here for a principle that underlies the argument in the stimulus. What is this argument? It concludes in essence that government should continue to devote resources to space exploration. Why? Because space exploration has had some good unexpected consequences. The principle seems to be something along the lines of "If something has had good unexpected consequences, we should keep doing it," and this is what (D) tells us. Hope this helps, and feel free to ask any further specific questions you might have.