A.one-third a timeB.one-third timeC.the one-third timeD.one-third the time

【答案】D

【解析】

考查数词及冠词。句意：他做这件事用了我用时的三分之一。数词one-third要放在名词前面，而名词time后面有定语从句it took me，故为特指，应在前面加定冠词the。故选D。