Please help me I only have a few minutes! Btw this is for Earth/Space Science! Given the following diagram, what product is produced at Location A?

1. Carbohydrates
2. Carbonic acid
3. Fossil fuels
4. Limestone

i wanna say limestone because if you lookup how limestone is formed its from animal skeletons (phytoplankton being the animal in this case) and as you can see the skeletons (sinking particles) are falling down to the bottom and limestone is formed at calm water and at the phytoplankton the arrow going straight down would in theory get enough light for organisms to make CaCO3 (calcium carbonate) and CaCO3 is needed to make limestone.  

why i'm not 100% percent sure of my answer is because im in 8th grade so im not yet learning this stuff because im taking physical science honors

i hope i am correct and if so i hope you get a good grade