Least costly way to reduce project time

Question

Problem: Consider this network and the time and cost data shown below. The least costly way (assuming a linear time cost relationship) to reduce project time by one day is to reduce time be one day for activity:

Normal Crash
Activity Time    Cost    Time Cost
1-2    6      100    5     200
1-4    15    200    12    600
2-3    5      100    4     190
3-4    5      150    3     370
5-3    6       80    5     185
5-4    8      300    7    360

a)    1-2
b)    1-4
c)    2-3
d)    Any of the above