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: Close observation of the response status both clinically and with beta - hCG values may indicate whether and when more agressive combination chemotherapy should be started . ( ABSTRACT TRUNCATED AT 250 WORDS )
### Output Text:
Close observation of the response status both clinically and with beta - hCG values may indicate whether and when more agressive combination chemotherapy should be started . ( ABSTRACT TRUNCATED AT 250 WORDS ) 