D
考查时态。根据for half a year可知应该使用完成时，再根据句意可知完成进行时符合句子的意思。Yet也和完成时连用。结合句意可知D正确。句意：半年来那位科学家一直在做那个实验，但他还没有成功。
The scientist __________ the experiment for half a year, but
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