In the following question, a sentence has been given in Active/Passive voice. Out of the four alternatives suggested, select the one which best expresses the same sentence in Passive/Active voice. I have kept all your old letters.

• Keeping of all your old letters has been done by me.
• I keep your letters which have become old.
• Your letters which have become old would be kept by me.
• All your old letters have been kept by me.