Which of the following environmental problems is the article talking about

A．he caught some bats and blinded them and released them
B．four of these blind bats found their way back
C．he recaptured the four returned bats
D．the stomachs of the blind bats found to be filled with flying insects

A．Pebbles tossed into the air make no sound that can be heard by human ears.
B．A bat will catch hundreds of moths in a single hour.
C．Insect-eating bats are totally blind.
D．The sonar system of bats is as good as man-made sonar.