Read these sentences from paragraph 4 of the speech to answer the question that follows: "[W]e have vowed that we shall not see it governed by a hostile flag of conquest, but by a banner of freedom and peace. We have vowed that we shall not see space filled with weapons of mass destruction, but with instruments of knowledge and understanding." How do Kennedy's two sets of opposing ideas contribute to his purpose in these sentences?

A. by creating a warning tone about the dangers of space exploration
B. by contrasting his motivations with those of other explorers
C. by emphasizing the potential dangers of failing to be first in space
D. by weighing the benefits of space exploration against its dangers