The main idea is the "key concept" being expressed. Details, major and minor, support the main idea by telling how, what, when, where, why, how much, or how many. Locating the topic, main idea, and supporting details helps you understand the point(s) the writer is attempting to express.
I do think it is C because it will always stay the same "Fixed"
Conventions are not an evaluative tool for a writer to use during the revision process.
