Further variance analysis F2

AD Ltd manufactures and sells a single product, E, and uses a standard absorption costing system.
Standard cost and selling price details for product E are as follows.
$ per unit
Variable cost 8
Fixed cost 2
10
Standard profit 5
Standard selling price 15
The sales volume variance reported for last period was $9,000 adverse.
AD Ltd is considering using standard marginal costing as the basis for variance reporting in future. What would be the correct sales volume variance to be shown in a marginal costing operating statement for
last period?
A $6,428 (A)
B $6,428 (F)
C $12,600 (F)
D $12,600 (A)

The answer by BPP is following. But it is pity I didnt understand this question and its answer. Specially this part of the answer Sales volume variance in terms of contribution = $9,000(A)/$5× $7 = $12,600 (A)

The sales volume variance in a marginal costing system is valued at standard contribution per unit, rather than standard profit per unit.
Contribution per unit of E = $15 ? $8 = $7
Sales volume variance in terms of contribution = $9,000(A)/$5× $7 = $12,600 (A)