46 points!! 1) What does “these" refer to? Choose the words that complete the sentence.

Recently, to address this problem, researchers have been setting up recorders in rainforests to capture the calls of the animals. Listening to these is like putting a stethoscope to a person's heart, though scientists don't hear just one animal at
a time. When they listen, they hear dozens of creatures
simultaneously lending their voices to the larger orchestra of rainforest sounds.
Listening to is like putting a stethoscope to a person's heart, though scientists don't hear just one animal at a time.

Listening to is like putting a stethoscope to a person’s heart, though scientists don’t hear just one animal at a time.

a- the researchers
b- the calls
c - the musicians
d- the problems