variance – sales volume contribution variance

A company operates a standard marginal costing system. Last month the company sold 200 units more than it planned to sell. The following data relate to last month:

selling price per unit – standard – $40 and actual $38
Variable cost per unit – standard – $30 and actual $29

What was the favourable sales volume contribution variance last month ?

A.$1600 B.$1800 C.$2000 D.$2200 ( answer given is C)

My working :
Budget 200 units x $10 =$2,000 my answer is c.
(selling price : standard $40 – variable cost per unit $30= $10 standard profit for marginal cost) , $10 profit for standard marginal cost whether is correct ?

and the question did not give actual unit therefore my working directly used the budget unit multiplied by the standard profit marginal cost per unit whether is correct ?

thank you