Concerns for the environment have led chemists to develop plastics that are degradable. All degradable plastics, which are potentially useful packaging materials, need just the right conditions to break down. Some need exposure to sunlight, some need to be buried in soil and some need to be submerged in water. It should be cautioned that some degradable plastics leave residues of unknown toxicity. If all of the statements above are true, which one of the following must also be true? ['Some materials that need to be buried in soil to break down leave residues of unknown toxicity.', 'Some degradable plastics need both sunlight and submersion in order to break down.', 'Some materials that are potentially useful for packaging leave residues of unknown toxicity.', 'Some materials that leave residues of unknown toxicity are not degradable plastics.']