A greenhouse is used to protect plants from the cold.
Greenhouses are made with glass , fiberglass , polyethylene, or vinyl coverings.
Something made of glass , fiberglass , polyethylene, or vinyl  is used to protect plants from the cold.
What protects plants from the cold?
Options:
[1] Something made of fruit or vegetables
[2] series of enzyme-mediated reactions
[3] Something made of glass, fiberglass, polyethylene, or vinyl
[4] Something whose growth is discouraged by heavy wet soils
[5] keeping food fresh for long periods of time
[6] Something made of brick
[7] Something made of metal
[8] gas layer that surrounds earth