Final answer:
C₂H₄ is primarily used in the manufacture of polyethylene plastic.
Step-by-step explanation:
C₂H₄, also known as ethylene, is primarily used in the manufacture of polyethylene plastic. It is a basic raw material in the production of various compounds used in the polymer, petrochemical, and plastic industries. Over 135 million tons of ethylene were produced worldwide in 2010 for this purpose.