Prove or disprove each of the following:
(a) For sets A, B, C ⊂ U, A ∩ C = B ∩ C ⇒ A = B.
(b) For sets A, B, C ⊂ U, A U C = B U C ⇒ A = B.
(c) For sets A, B, C ⊂ °U,
[(A ∩ C = B ∩ C) ∧ (A U C = B U C)] ⇒ A = B.
(d) For sets, A, B,C ⊂ U A ∆ C = B ∆ C ⇒ A = B.