Which of these is a biotic factor of an aquarium ecosystem?

a: algae growing on the glass
b: gravel on the bottom of the aquarium
c: temperature of the water
d: amount of oxygen in the water

because natural gas is colorless, odorless, and tasteless, distributors add mercaptan (a chemical that smells like sulfur) to give natural gas a distinct unpleasant odor (it smells like rotten eggs). they locate the types of rock that are likely to contain natural gas deposits.