What is the name of an ancient volcano whose top has eroded away?

guyot
ridge
seamount
trench

Guyot

Guyot is the name of an ancient volcano whose top has eroded away. Guyot is an under water volcanic mountain that has a flat top. Guyot is most commonly found in the Pacific Ocean. A Guyot remains inside the sea water the top represents a past volcanic event.   

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds