please explain each question

Exercise 3 (16 points total, 2 points for each blank) In this problem, let REF(x.i) DEF(x.k) denote that the linker will associate a reference to symbol x in module i to the definition of x in module k. For each example that follows, use this notation to indicate how the linker would resolve references to the symbol in each module. If there is a link-time error (rule 1), write "error". If the linker arbitrarily chooses one of the definitions (rule 3), write "unknown". (1) (1.1) REF(main.1) DEF (1.2) REF( main.2) DEF( (2) (2.1) REF(main.1) DEFI (2.2) REF( main.2) DEFI (3) (3.1) REF(x.1)DEF( (3.2) REF(x.2)DEF (4) (4.1) REF(x.1)DEF( (4.2)REF(x.2)DEF