May I use only C(without use the superset of C)? Will my app become unstable?
You can use straight ANSI C as long as your app doesn't use require any UI interaction (any user input or window output), and runs less than 20 seconds.
For instance, you can write an app that makes a posix network connection, downloads some data and plays a PCM sound waveform without writing any Objective C (or maybe 3 or 4 lines of it), other than in the templates.
Otherwise you will need to use some Objective-C to access the UIKit frameworks for any UI (touches, accelerometer, windows, views, etc.), and the main run loop.