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: Angiotensin - converting enzyme ( ACE ) inhibitors should be used first as they are safer , do not require blood level monitoring , modify progression of disease , relieve symptoms , improve exercise tolerance and reduce mortality .
### Output Text:
<span class="chemical">Angiotensin</span> - converting enzyme ( ACE ) inhibitors should be used first as they are safer , do not require blood level monitoring , modify progression of disease , relieve symptoms , improve exercise tolerance and reduce mortality . 