複選題(3) In a stack structure, let u denote a push operation and o denote a pop operation. After a sequence of stack operations, consisting of 3 pushes and 3 pops, an input sequence 123 may change its order. For example, after uouuoo is performed, 123 will become 132. Which sequence(s) cannot be obtained from input 123 by stack operations? (A) 213 (B) 231 (C) 312 (D) 321.