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 , used to treat hypertension and congestive heart failure , were introduced in Europe in the middle of the eighties , and the use of these drugs has increased progressively .
### Output Text:
<span class="chemical">Angiotensin - converting enzyme ( ACE ) inhibitors</span> , used to treat hypertension and congestive heart failure , were introduced in Europe in the middle of the eighties , and the use of these drugs has increased progressively . 