In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The occurrence of PVC ' s correlated significantly with the fall in serum K + observed r = 0 . 72 , p less than 0 . 001 .
### Output Text:
The occurrence of PVC ' s correlated significantly with the fall in serum <span class="chemical">K</span> + observed r = 0 . 72 , p less than 0 . 001 . 