Why variable cost is taken to calculate C/S ratio

Dear John,

In PM MCQ Test (Question 1), contribution to sales ratio is defined as 30% and the Variable cost is defined as $28 per unit.

In Answer part, while calculating the Selling Price per unit, why the c/s ratio is calculated from the % of variable cost instead, as per the understanding received through lectures, the c/s ratio is the linkage between contribution and selling price.