Instead of writing the flat sign before every B note on the staff, one flat sign can be written after each clef. This means that all B notes on the staff are played as Bb, even though there is no flat sign written before them. This is the key signature for the key of F major. There is one flat sign after each clef. Exercise 78 is in the key of F.
This song is in the key of F. All B notes in both parts are played as Bb.