What will be the "main purposes for your your visit" in the visa form for attending IEEE conference?

I have to go to china on 11 june for presenting a paper in the "International conference on micro/nano optical engineering". Today I went to embassy and I filled up the main purposes as "Other- for conference. I had the Chinese invitation letter from the conference committee (which is soft copy, mailed by them). But in the embassy, they asked for hard copy and talking about some local govt. letter. I think they thought the conference as a governmental conference, but is for only educational purposes. So what will be the "main purposes for visit" in the visa form? Attending this conference is very important for my study. What should I do now? Please reply soon. Thanks.