Eli Fry-Son or Grandson of David Samuel Mast?

The 1880 United States Census for Walnut Creek, Holmes, Ohio citing sheet 429C, family 10, the household of David Mast lists an Eli Fry, age 2, as a son of David Mast.  Adopted is written in the column for profession. I was curious about this and wondered how Eli fit into the family.  I did a little research on Eli.

According to "Ohio, County Births, 1841-2003," index and images, FamilySearch (https://familysearch.org/pal:/MM9.3.1/TH-1951-23099-39857-27?cc=1932106 : accessed 21 Nov 2013), > image 1 of 1, Elias Fry was born 14 September 1878 to Noah Fry and Christina Mast in Holmes County, Ohio.  

I knew that Christina Mast was the daughter of David Mast and Anna Nancy Livengood, but further research was needed.  I tried to find a marriage certificate for Noah Fry and Christina Mast and was unsuccessful.  Another search was done for a death record of Noah Fry and Christina Mast and Christina Fry with negative results. These findings added to my determination to learn more about Eli.

I have more questions than answers.  Did Noah Fry and Christina Mast ever marry? Why was Eli listed as adopted son of Christina's parents two years after his birth? No Christina was in the David Mast household in 1880.  There was a Dinah listed in the census, is Dinah Christina?  When did Christina and Noah die?

Even though I have more research to complete on Eli Fry, I know that Eli was the grandson of David Samuel Mast.  Was he adopted?  I have yet to prove that.