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: A subjective assessment of sexual behavior in the study and control groups was carried out , using parameters such as sexual desire , sexual excitement , and frequency of sexual relations .
### Output Text:
A subjective assessment of sexual behavior in the study and control groups was carried out , using parameters such as sexual desire , sexual excitement , and frequency of sexual relations . 