. What does the word "trill" most likely mean, based on its context in stanza 2 below?
"The caged bird sings/ With a fearful trill/ Of things unknown/ But longed for still/ And
his tune is heard"
a. Trembling or quivering sound made by a bird or insect
b. Another animal of a similar species
c. A peaceful song sung by a bird or insect
d. Strong, confident tone