【正确答案】 D

【答案解析】[解析] 画线词invaluable为形容词，意为“无法估计的，十分宝贵的”，与D项extremely useful(非常有用的)意思相近；例：Newton's law of universal gravitation has been extremely useful. 牛顿的万有引力定律是极其有用的。A项simply useless意为“无用的”；例：They are simply useless. 它们简直没有用。B项really practical意为“确实很实用的”；例：It is not really practical to start from ground zero in such a competitive job market. 因为面对竞争如此激烈的劳务市场，一切从头开始不太现实。C项very little意为“几乎没有”；例：It took very little time. 做那件事花了很少的时间。