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 (强大的) 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 b下面是一个验证唾液淀粉酶专一性的一个实验设计。 [原理提示](1)滴加本迪森试剂如果出现黄色，证明有还原性糖存在。(2)唾液淀粉酶能将淀粉水解为麦芽糖，但不能将蔗糖水解成为单糖，麦芽糖、果糖、葡萄糖具有还原性，淀粉和蔗糖则不具有还原性。 [操作与记录] 加入物质 试管号 1 2 3 4 1%淀粉溶液(滴) 4 — 4 — 2%蔗糖溶液(滴) — 4 — 4 稀释的唾液(滴) — — 1 蒸馏水(滴) 1 1 — 恒温处理15min 37 ℃ 37 ℃ 37 ℃ 37 ℃ 本迪森试剂(滴) 1 1 1 1 沸水浴(min) 3 3 3 3 现象 不变黄 不变黄 变黄 不变黄 [结果分析] 请根据以上表格分析下列问题： (1)表中3号和4号试管各留下一处空白，3号试管空白处的操作应该是__________________，4号试管空白处的处理应是__________。 (2)实验中1号和2号试管起__________作用。 (3)实验结果说明________号试管内含有还原性糖。 (4)本实验中保持试管温度为37 ℃的理由是__________。 (5)试从结果分析，说明这个设计为什么能说明唾液淀粉酶的专一性？________________________________________________________________________________________________。