Why is an ecosystem's net primary production (npp) lower than its gross primary production (gpp)?
a. npp reflects the loss of matter due to respiration by the primary producers.
b. npp reflects the gain of oxygen due to respiration by the primary producers. c. gpp reflects the loss of matter due to respiration by the primary producers.

Hi. I think it would be A. Heres how NPP and GPP work:

NPP stands for net primary productivity and GPP means gross primary productivity. GPP is basically the entire amount that an ecosystem or area has. NPP is the amount minus the amount lost to the environment. You might already know that as you go up the food chain energy is lost in respiration and doesn't get absorbed by the thing that eats it. Therefore, the NPP=GPP-lost energy. 

If you have any questions about this please comment :) sorry if this was unclear at all.

The light is great source of energy