Which of the following is not a valid postfix expression? A. 5 4 + B. 6 5 4 + - C. 4 + 5 D. 8 2 + 2 / E. all of the above are valid postfix expressions

To determine which of the following is not a valid postfix expression, let's analyze each option:

1. Option A: 5 4 +

   • This is valid. It adds 5 and 4.

2. Option B: 6 5 4 + -

   • This is valid. The expression evaluates as follows:
     • 5 4 + results in 9.
     • Then 6 9 - results in -3.

3. Option C: 4 + 5

   • This is invalid as it does not follow postfix notation. In postfix, the operator should come after the operands.

4. Option D: 8 2 + 2 /

   • This is valid. The evaluation would be:
     • 8 2 + results in 10.
     • Then 10 2 / results in 5.

5. Option E: All of the above are valid postfix expressions

   • This is incorrect because option C is not valid.

Final Answer

C. 4 + 5 is not a valid postfix expression.