[解析]
答案:Unfortunately/Unluckily
考查副词.根据所给的汉语提示"不幸的是"和句子结构可知,此处应用副词作状语修饰句子,"不幸的是"的英语是unfortunately/unluckily,例如:Unfortunately/unluckily, attempts to crossbreed it with other potatoes have been unsuccessful.不幸的是,将它与其他土豆品种杂交的尝试均告失败故填Unfortunately/Unluckily.