3, definitely. The other answers are all harmful to the environment or flat-out wrong. At least it gets the economy going and lets more people make a living... even if it's hurting the ozone.
Multicellular organisms can be much larger and more complex. This is because the cells of the organism have specialised into many different types of cells such as nerve cells, blood cells, muscle cells all performing different functions.
The excretory system will help remove extra water and eliminate it as urine.
C. the interaction between dominant and recessive genes. 100%