题目

根据短文理解，选择正确答案。 Once when I was facing a decision that involved high risk, I went to a friend. He looked at me for a moment, and then wrote a sentence containing the best advice I've ever had: Be bold and brave — and mighty forces will come to your aid. Those words made me see clearly that when I had fallen short in the past, it was seldom because I had tried and failed. It was usually because I had let fear of failure stop me from trying at all. On the other hand, whenever I had plunged into deep water, forced by courage or circumstance, I had always been able to 19．磷单质及其化合物有广泛应用．（1）三聚磷酸可视为三个磷酸分子之间脱去两个水分子的产物，三聚磷酸钠（俗称“磷酸五钠”）是常用的水处理剂，其化学式为Na5P3O10．（2）次磷酸钠（NaH2PO2）可用于化学镀镍．①NaH2PO2中P元素的化合价为+1．②化学镀镍的溶液中含有Ni2+和H2PO2-，在酸性条件下发生下述反应，请配平：2Ni2++1H2PO2-+1H2O═2Ni++1H2PO3-+2H+（3）磷酸钙与焦炭、石英砂混合，在电炉中加热到1 500℃生成白磷，反应为2Ca3（PO4）2+6SiO2$\frac{\underline{\;\;△\;\;}}{\;}$6CaSiO3+P4O1010C+P4O10$\frac{\underline{\;\;△\;\;}}{\;}$P4+10CO若反应生成31g P4，则反应过程中转移的电子数为5NA（用NA表示阿伏加德罗常数的值）．