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: BACKGROUND : The involvement of water - soluble carotenoids , crocins , as the main and active components of Crocus sativus L . extract in learning and memory processes has been proposed .
### Output Text:
BACKGROUND : The involvement of water - soluble <span class="chemical">carotenoids</span> , <span class="chemical">crocins</span> , as the main and active components of Crocus sativus L . extract in learning and memory processes has been proposed . 