Please choose the correct ending to complete the given sentence: Pets and Animals: [header] How to prevent tear staining in dogs [title] Understand tear stains. [step] Tears contain a chemical called porphyrin that turns rust-colored when exposed to the air. A process called oxidation causes an eventual color stain to a dog's fur.

Ending1: This color is then absorbed through the coat, leaving behind no visible need to repair. Overnight stains are considered a fair sign and your dog may be able to fight eye infection. Ending2: Watch out for extreme tear stains, whether they're flare stains, extensive grime on the skin, or areas that are not porphyrin resistant. Pure oxidation relies heavily on the presence of nitrates in the skin so that eventually the stain will disappear. Ending3: Just like hair dye, it takes a while for the porphyrin to stain fur. Tear staining is a result of tears not being wiped away before the fur gets stained. Ending4: Though stain removal is quick and easy, it costs significantly more than professional bleaching to stop the stain. [substeps] The chemical can remove minor stains, acidic stains, and others, but it can generally not be removed in a single stroke.

Answer format: ending1/ending2/ending3/ending4