关于产生感应电流的条件，下列说法正确的是              （）        

增加：在缺词处加一个漏字符号( )，并在其下面写出该加的词。

删除：把多余的词用斜线(＼)划掉。

修改：在错的词下划一横线，并在该词下面写出修改后的词。

注意：1）每处错误及修改均仅限一词。

2）只允许修改10处，多者（从第11处起）不计分。

    There was an art festival in our school last weekend. It had been held for five years and our headmaster plan to make it a feature of our school. I was so excited, for it was first time for me to join in.

    Early in the morning, there were such many people coming to our school, made it so lively. The students gave a warm welcome to the guest. I walked around or appreciated the paintings. All the works were from the students and I was so impressing by them. There were also some handcrafts, that were so creative. Though we were busy with our study most of the time, but some students still managed to develop their hobbies. I should learn to them and develop a hobby as well.

将矩形纸片ABCD按如图所示的方式折叠，    得到菱形AECF．若AB=3，则BC的长为(     )