阅读下面材料, 从A、B、C、D四个选项中选出最佳选项。

    Jumping rope(绳子)is a sport any student can learn to do．You can jump alone or with a friend．You can also help turn a long rope for many others to jump over．If you jump alone, you will need a short rope which should be just long enough for you．

    A rope should be turned slowly．It must also be turned high enough to let you jump safely．Turning the rope safely needs careful timing．In rope jumping, timing means turning the rope slowly enough so that you can jump over it at the right time without its hitting your feet．

  You will notice that when you jump rope, you breathe(呼吸)faster and your face has a healthy colour．But, if you jump too many times at first, you will get too tired．You must learn when to stop for rest(休息)before you jump again．As you practice, you can increase the number(次数)of times you jump without resting．

某兴趣小组为探究碱性条件对酶活性的影响，设计了如图所示的实验方案（图中序号表示各实验步骤次序）：

请回答下列问题：

（1）实验时将试管置于37℃水浴中保温是因为____ ；

（2）B的实验步骤②和③能否对调？____ ；

（3）预计加碘液后可能出现的实验现象是A试管的溶液____ ，B试管的溶液变蓝．

实验发现A试管的结果与预计相符，B试管的溶液却不显蓝色．他们猜想：可能是唾液淀粉酶在碱性条件下也可以水解淀粉．为此，他们重做实验，只在步骤⑤中用斐林试剂（在加热条件下遇淀粉的水解产物会产生砖红色沉淀）代替了碘液，结果A试管中产生砖红色沉淀，B试管中没有产生砖红色沉淀，说明唾液淀粉酶在碱性条件下____ （选填：“能”或“不能”）水解淀粉．通过进一步查阅资料知道，碘能与氢氧化钠溶液反应，所以第一次实验时B试管的溶液不显蓝色．