Here's why:

* Pure Substance: A pure substance is made up of only one type of molecule. Polyethylene is a polymer made up of repeating units of ethylene molecules (C2H4).

* Homogeneous Mixture: A homogeneous mixture has a uniform composition throughout. While different grades of polyethylene may have varying chain lengths and densities, they are still considered homogenous because the polyethylene molecules are evenly distributed.

* Heterogeneous Mixture: A heterogeneous mixture has a non-uniform composition. Polyethylene does not exhibit this characteristic.

Important Note: While different grades of polyethylene exist (like high-density polyethylene (HDPE) and low-density polyethylene (LDPE)), these variations are still considered pure substances as they are composed of the same type of molecule with different degrees of branching or chain length.