In order to estimate the gross primary productivity (GPP) of a pond containing phytoplankton and zooplankton, a student collected water samples from the pond in two bottles labelled X and Y . The dissolved oxygen (DO) of the samples immediately after collection was $8 \mathrm{mg} \mathrm{ml}^{-1}$. The DO content when measured after 24 hours of incubation was as follows:
Bottle X: $2 \mathrm{mg} \mathrm{ml}^{-1}$ after incubation in dark
Bottle Y: $18 \mathrm{mg} \mathrm{ml}^{-1}$ after incubation in light
GPP of the pond is:

• A

  $6$
• B

  $10$
• Correct answer
  C

  $16$
• D

  $20$

Hence, the correct answer is option: 'C'.