Process Costing

This is a BPP MCQ.
A manufacturer produces Product X from three input raw materials, P, Q and R. There is some loss in the process, usually about 5% of input. Results for the most recent month are as follows.

Input
5000kg of P, cost of $15000
2000kg of Q, cost of $8000
1000kg of R, cost of $8000

Output
7500kg of Product X

Which one of the following performance measurements is accurate?

A) Rate of wastage = 6.7%
B) Average cost of input = 4.13 per kg
C) Output: input conversion ratio = 6.25%
D) Materials yield variance = $1938(A)

The correct answer is B. I think this should be average cost of output.
Help me, Sir.