put to work vs retain

put to work

verb
  • to put to use 

  • Give a job; Force to work (even if it is make-work) 

retain

verb
  • To keep in possession or use. 

  • To employ by paying a retainer. 

  • To hold back (a pupil) instead of allowing them to advance to the next class or year. 

  • To keep in one's pay or service. 

  • To hold secure. 

How often have the words put to work and retain occurred in a corpus of books? (source: Google Ngram Viewer )