Choose the answer which correctly completes the sentence

Choose the answer which correctly completes the sentence.

The price of sending a first-class letter within the United States … by 1500% since the turn of the century, when it … only 2 cents.

1. 
   Is increasing/had cost B) increased/used to cost C) increases/would have cost D) has increased/cost
   

24. 
    Choose the answer which correctly completes the sentence.
    

I believe you … this computer programme in your previous job, so you … it difficult at times. Please don`t hesitate to ask me for help.

1. 
   Didn`t use/might find
   
2. 
   Haven`t used/must find
   
3. 
   Couldn`t use/have found
   
4. 
   May not use/will find
   

25.