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: More than 20 % ( 47 . 61 - 67 . 2 % ) reduction in central retinal thickness was noted in all patients with cystoid macular edema at 6 months , 1 year , 2 years , and 3 years postimplant .
### Output Text:
More than 20 % ( 47 . 61 - 67 . 2 % ) reduction in central retinal thickness was noted in all patients with cystoid macular edema at 6 months , 1 year , 2 years , and 3 years postimplant . 