pondering vs preoccupation

pondering

noun
  • An act of rumination or cogitation. 

preoccupation

noun
  • The state of being preoccupied or an idea that preoccupies the mind; enthrallment. 

  • The act of occupying something before someone else. 

How often have the words pondering and preoccupation occurred in a corpus of books? (source: Google Ngram Viewer )