Answer:
A.Earth's early atmosphere lacked oxygen, which multicellular organisms need to break apart food
Step-by-step explanation:
One important feature that led to the development of multicellular organisms was the rise of oxygen-producing photosynthetic bacteria. Prior to this development, the unicellular organisms (anaerobes) then present on Earth obtained energy from inorganic fuels.
However, with the presence of oxygen in the atmosphere, a shift was observed as organisms that could obtain energy from the reduction of energy became predominant. This unicellular aerobes later paved way for the evolution of multicellular aerobes.