D
Journalist: Many people working on difficult problems in mathematics report going to sleep without a solution, but upon awaking discover they have a solution in mind. This phenomenon occurs among all age groups past infancy. Which one of the following is most strongly supported by the journalist's statements?
A: Certain types of mathematical problems cannot be solved while one is consciously seeking a solution.
B: The ability to carry out mental processing gradually develops during infancy.
C: Everyone's mind works unconsciously on difficult problems while sleeping.
D: Consciously seeking a solution is not the only mental process by which one can solve a mathematical problem.