Receivables period at the period end

Hello Sir,

Here for calculation of receivable period why we don’t consider the average receivable
and when we have to consider that.

For a retailer, let  
S   = Sales 
P   = Purchases 
O   = Opening inventory 
C   = Closing inventory 
D   = Opening receivables 
R   = Closing receivables 
Which of the following is the best expression for receivables days at the period end? 
A  [D + R]/P × 365 
B  [D + R]/S × 365 
C  R/S × 365 
D  D/S × 365 

Answer is

C