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: After excluding case reports , the following groups were constructed : ( a ) short - term administration , after valve insertion ( n = 212 ) ; ( b ) short - term , perioperative ( noncardiac ) / periprocedural ( n = 114 ) ; ( c ) long - term , due to intolerance to OAC ( n = 16 ) ; ( d ) long - term , in pregnancy ( n = 10 ) .
### Output Text:
After excluding case reports , the following groups were constructed : ( a ) short - term administration , after valve insertion ( n = 212 ) ; ( b ) short - term , perioperative ( noncardiac ) / periprocedural ( n = 114 ) ; ( c ) long - term , due to intolerance to OAC ( n = 16 ) ; ( d ) long - term , in pregnancy ( n = 10 ) . 