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Simplify: $$b\left( a+1 \right)-a\left( b-1 \right)$$． 
Overseas Competition->Knowledge Point->Calculation Modules->Basic Concepts of Equation->Equivalent Substitution->Algebraic Expressions
$$b\left( a+1 \right)-a\left( b-1 \right)=ab+b-ab+a=b+a$$, so the answer is$\text{A}$$. 