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However , the study on H2S has been hindered due to the lack of controllable H2S releasing agents that could mimic the slow and moderate H2S release in vivo .
 Chemical: H2S; Chemical: H2S; Chemical: H2S
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: H2S; Chemical: H2S; Chemical: H2S