Answer:
b.$77
Explanation:
beginning inventory 5 units
purchase september 4th 8 untis
sale for 6 units:
inventory after first sale: (available at second sale)
beginning used.
purchase september 4th  7  untis at $6
September 25th Sale of 12 units
inventory used for second sale:
September 4th  7  untis at $6
September 15th  5 units at $7
total COGS for the sale: $77