A company sells wooden chairs at a selling price of IDR $30$ thousand per chair. The purchase cost from the supplier is IDR $16$ thousand per chair. The company gives sales commissions to salespeople of $10\%$ of the amount sold and incurs shipping costs of IDR $1$ thousand per chair.

In order to support its operational activities, the company rents a coworking space with a rental fee of IDR $5$ million per month and other operational costs $($salaries$,$ utilities$)$ of IDR $10$ million per month. The company does not have other costs outside operational activities.

If the company plans to change the office rental fee scheme to be variable on the number of sales, where the rental fee paid is IDR $5$ thousand per chair sold, then what is the number of seats sold so that there is an indifference to the old fee scheme? $($Note: Salary and utility costs still arise$)$

a$.667$ units

b$.1,000$ units

c$.1,667$ units

d$.2,000$ units