For example, some organic type waste materials are now believed to pose {TAIL ~ possible valuable sources} of {HEAD ~ methane} as naturally occurring fossil fuels are depleted 
For example , some organic type waste materials are now believed to pose { TAIL ~ possible valuable sources } of { HEAD ~ methane } as naturally occurring fossil fuels are depleted
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH