Sentence:  Christiana Securities Company and Delaware Realty + Investment Company , major stockholders in du Pont , and the stockholders of Delaware were dealt with specially by provisions requiring the annual sale by a trustee , again over a ten year <WSD>period</WSD> , of du Pont 's General Motors stock allocable to them , as well as any General Motors stock which Christiana and Delaware owned outright . 
Meaning of the word marked with <WSD> tags:  an amount of time