Sentence:  One of the important aspects of this study was to determine the <WSD>oxygen</WSD> transfer relationships of the mechanical aerator . 
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