Expressions: The combination of numbers, symbols and operators.  (1) A single number or a single letter can be considered an algebraic expression.  (2) Equations and inequalities are NOT expressions.  Which of the following is \textbf{NOT} an expression? 
选项为:A:$$3$$  B:a  C:$$a+b=a+b$$  D:$$b+1+a$$  请回答其中一个选项.