A manufacturer can produce $12000$ bearings per day. The manufacturer received an order of $8000$ bearings per day from a customer. The cost of holding a bearing in stock is $Rs$. $0.20$ per month. Setup cost per production run is $Rs$. $50 0$. Assuming $300$ working days in a year, the frequency of production run should be

1. $4.5$ days
2. $4.5$ months
3. $6.8$ days
4. $6.8$ months