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: Cardiovascular diseases ( CVDs ) are the major health problem of advanced as well as developing countries of the world .
### Output Text:
Cardiovascular diseases ( CVDs ) are the major health problem of advanced as well as developing countries of the world . 