The gas may be used as {TAIL ~ a single type} thereof or as combinations of {HEAD ~ different gases} 
The gas may be used as { TAIL ~ a single type } thereof or as combinations of { HEAD ~ different gases }
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD