Sentence:  The polyethylene cover reduced the <WSD>oxygen</WSD> transfer rate by 10 per cent , indicating that the maximum oxygen transfer is at the rotor rather than through the surface . 
Meaning of the word marked with <WSD> tags:  a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust