At what project duration do we achieve the lowest total

Question

You have collected the data for a Time-Cost CPM Scheduling model analysis. The time is in days and the project direct costs are given below.

The network for this project is given below.

The indirect costs for the project are determined on a daily duration basis and are given below:

Total indirect costs for 16 days of project duration: $ 600
Total indirect costs for 15 days of project duration: $ 400
Total indirect costs for 14 days of project duration: $ 200
Total indirect costs for 13 days of project duration: $ 100

At what project duration do we achieve the lowest total project cost(i.e., direct plus indirect costs)? 
(a) 16 days 
(b) 15 days 
(c) 14 days
(d) 13 days