This kind of questions are frequently asked here. You may search a little in the forum and could find many threads which discussed about this. Easiest way to solve these kinds are by drawing an object graph in a paper. Not getting your question though...
//at line 9 c.aob and c.aob both are referring to null:
You are talking about the same reference (i.e: c.aob) here?
but the object to which B and c are referring to still can be accessed by a.aob as in line4
Yes, the reference "a" is still alive (I guess you meant b, not B..).