It is not c or d. I think it is a.
A leaf is made up of many layers and is surrounded by two. Upper epidermis, mesophyll, bundle sheath, vein, mesophyll, lower epidermis, and guard cells.
I believe the appropriate answer is a cooling-off period. The cooling off period us described as the state of returning to the offender's usual way of life between homicides. The key distinction between the serial killers and mass or spree killers is this emotional cooling off period in which the killer blends back into his/her normal life.
Carbon is an element
Oxygen is an element
The answer is C: a pollutant.
