问题如下：
In March 2017, Alex bought 500 units of pens with $2 cost per unit. In April 2017, Alex bought 600 units of pens with $3 cost per unit. In 2017,Alex sold 1000 units in $5 per unit. When he uses Weighted average method , the cost of good sold in 2017 will be:
选项：
A. 2.545
B. 1532
C. 2545
解释：
C is correct. 2800/1100=2.545;2.545*1000=2545
很奇怪，之前有道题在计算weight average cost的时候，只是把卖出的部分计算在内，按照这道题的条件，也就是wac=（500*2+500*3）/（500+500），而这道题就是把全部的采购数量算在内也就是500+600，计算wac时到底算所有的数量，还是只算卖出的数量？
