Hello! My name is D.T. I'm a monster. I'm a boy. I have a big head and a small body. I have one eye, two mouth, three nose and four ears. My face is green.My eyes are blue. I have two arms and three legs. I have three hands and nine fingers. My hands are yellow. I feel very cool.

实验是科学探究的重要途径，请根据如图所示的实验回答有关问题．

（1）A实验中如果实验数据小于21%，可能的原因是____ （写出一点）；

（2）如图B所示，用pH试纸测定NaOH溶液的pH时，先将pH试纸用水湿润，所测得的结果将____ （填“偏大”、“偏小”或“不变”）；

（3）C实验中铁丝在氧气中燃烧的现象为____ ；

（4）D实验中操作有①计算产率、②过滤、③溶解、④蒸发，正确的操作顺序为____ （填序号），若要除去粗盐中的氯化镁，选择的除杂质试剂不能用KOH代替NaOH，理由是____．