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: Five hours after starting the infusions , a study of the pituitary responses to LH - releasing hormone ( LH - RH ) was carried out .
### Output Text:
Five hours after starting the infusions , a study of the pituitary responses to LH - releasing hormone ( LH - RH ) was carried out . 