1. Show that D (A v B) --> (DA v OB) is invalid in K by building a countermodel 2. Show that (A V OB) --> D(A v B) is valid in K by an axiomatic proof 3. Show that (A & OB) ---> 0(A & B) is invalid in K by building a countermodel 4. Show that 0(A & B) --> (OA & OB) is valid in K by a semantic validity proof