C: I'm sorry I didn't write to you earlier. I have been busy since last week.
"since" does not go with "was." It goes with "has been," or a derivative of it. You could get rid of "since" and just say "I was busy last week." but that wouldn't explain why you haven't replied this week. Hence, you've been busy up until now, including last week, and you couldn't make time to write a letter. You have been busy since last week.
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BullitNutz, University of Central Florida - Junior, BA: English Program.