谁能帮我看看part2的这道题。。。。

Folsom Fashions sells a line of women’s dresses. Folsom’s
performance report for November follows.
The company uses a flexible budget to analyze its
performance and to measure the effect on operating income
of the various factors affecting the difference between
budgeted and actual operating income.
                                          Actual                         Budget
Dresses sold                  5,000                         6,000
Sales                             $235,000                  $300,000
Variable costs                (145,000)                (180,000)
Contribution margin (CM) 90,000                 120,000
Fixed costs                      (84,000)                   (80,000)
Operating income               $ 6,000                    $ 40,000

Folsom’s sales price variance for November is
A. $30,000 unfavorable.
B. $18,000 unfavorable.
C. $20,000 unfavorable.
D. $15,000 unfavorable.

答案是 (D) is correct. The sales price variance is the actual number of units sold (5,000), times the difference between
budgeted selling price ($300,000 ÷ 6,000) and actual selling price ($235,000 ÷ 5,000).
$50 - $47 x 5,000 = $15,000 U

可我怎么觉得不对啊，不是应该用cm的差去乘么，（20-18）×5000=10，000 可是又没有这个答案。谁能帮我解释一下啊？