Task: Question Answering
Input (English): Passage: During a choir practice two days before Christmas, village organist Franz Gruber is worried to hear unusual sounds from the church organ and suspects the bellows. One of Gruber's sons discovers mice in the pipes of the organ and the mice have chewed up parts of the organ. Without the organ the church choir cannot perform the rehearsed Bach piece because the music was written to be performed with an organ.
Accompanied by both his sons, Gruber travels to Salzburg, hoping to buy spare parts and then mend the organ when returning to the village. The party is caught in a snow storm and the spare part is lost on the way home but pastor Joseph Mohr is simply thankful they are unharmed.
Pastor Mohr (with a little help from the bell-ringer Otto) writes the lyrics for a song and the next morning he brings it to Gruber, asking him to compose a melody for the lyrics. With some inspiration from his wife, Gruber sets music to Mohr's words. At church Gruber and Mohr presents "Silent Night" performed a cappella by the choir. 
Question: Who are the people related to Franz in the party that is caught in a snow storm?
Output (English): his sons.