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: In the last few years delayed reactions several hours after the injection of radiographic and contrast materials ( PRC ) have been described with increasing frequency .
### Output Text:
In the last few years delayed reactions several hours after the injection of radiographic and <span class="chemical">contrast materials</span> ( <span class="chemical">PRC</span> ) have been described with increasing frequency . 