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: METHODS : Standard pattern electroretinograms ( P - ERGs ) and standard pattern visual evoked potentials ( P - VEPs ) were done in 25 eyes of 25 patients who had myopic PRK for an attempted correction between 5 . 00 and 15 . 00 diopters ( D ) ( mean 8 . 00 D ) .
### Output Text:
METHODS : Standard pattern electroretinograms ( P - ERGs ) and standard pattern visual evoked potentials ( P - VEPs ) were done in 25 eyes of 25 patients who had myopic PRK for an attempted correction between 5 . 00 and 15 . 00 diopters ( D ) ( mean 8 . 00 D ) . 