Scientists say that that the global rate of terrestrial net ecosystem production (NEP)is approximately 56.4 Pg of carbon per year.However,there may be 30% or more uncertainty in these rates.What does that mean?

A) That the NEP is exactly 56.4 Pg of carbon per year and that it does not fluctuate
B) That the global NEP is inferred from data, and that there may be plus or minus 30% of 56.4 Pg of carbon per year
C) That the global NEP is absolute, and that it will likely be higher than 56.4 Pg of carbon per year
D) That the NEP is not reliable because there is 30% or more uncertainty. Therefore we should not trust the estimate.