F3 inventory question ...thanks

A company’’s gross profit as a percentage of sales decreased from 28% in the year
ended 31 December 20X1 to 25% in the year ended 31 December 20X2.
Which of the following events most likely caused the decrease?
A An increase in sales volume
B A purchase in December 20X1 mistakenly being recorded as happening in January 20X2
C Overstatement of the closing inventory at 31 December 20X1
D Understatement of the closing inventory at 31 December 20X1
The answer is C, but why B is incorrect? many thanks~~~