You will notice that a sharp sign has been placed on the F line after each clef in Exercise 24. This is called a key signature. The key signature identifies the key of the song. One sharp (F#) indicates that the song is in the key of G, and that all F notes anywhere on either staff are played as F#. E.g., the D7 chords in bars 9 and 12, contain an F# note. Key signatures are discussed in more detail in Appendix 2.
The key of C major contains no sharps or flats, so the key signature is:
The key signature of G is:
No sharps or flats
It contains 1 sharp (F#)
Most of the previous chord progressions you have been given have been in the key of C major