Chemistry Riddles

I Am Present in Marshes
Riddle:
I am present in marshes And also in the rumen of cattle To stop global warming I do very little. What am I?
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Periodic Elements That Terrifies Criminals
Riddle:
When combined, these four periodic elements make up something that terrifies criminals?
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Occasionally Associated With Smelly Things
Riddle:
I am occasionally associated with smelly things. What state of matter am I?
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