15.The only strategic metal in which the United States is self-sufficient, molybdenum is a soft substance that can, paradoxically, be used to harden steel in alloys, as well as a lubricant, and, in trace amounts, in nutritional supplements to help prevent tooth decay.
C. in alloys to harden steel, as well as a lubricant and nutritional supplement in trace amount
D. in alloys to harden steel; it can also be used as a lubricant and in trace amounts, as a nutritional supplement
E. in alloys to harden steel; it can also be used as well as a lubricant and nutritional supplement in trace amounts
基本上 這個題目最難懂得地方在於 in trace amounts 是什麼意思阿?
在題目以一大堆標點符號迷惑後,真是把我給搞糊塗了!