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: The most striking effect of combining T + PCPA was a significant increase in attack frequency as well as a significant decrease in the latency to attack , particularly following physical provocation .
### Output Text:
The most striking effect of combining <span class="chemical">T</span> + <span class="chemical">PCPA</span> was a significant increase in attack frequency as well as a significant decrease in the latency to attack , particularly following physical provocation . 