What is bromthymol blue? why is the btb in the beaker green? answer sheet?

Bromthymol blue is a chemical that changes color to indicate the presence of oxygen or carbon iv oxide in a water sample. If oxygen is present it changes color from green to blue and if carbon iv oxide is present it changes from green to yellow. Therefore in this case where the BTB was green, it means there was no oxygen or carbon iv oxide in the beaker.