Bad debts and allowance for doubtful debts

Greetings everyone!

I’m new to this forum and didn’t know where to post my question so I’m posting it here.
It’s an MCQ question and my concept is a bit hazy.Here we go;

Q: Before any end-of-year adjustments had been made, the trial balance of a business at 31 May 2011 included the following.

trade receivables 13 400dr
provision for doubtful debts 500cr

At 31 May 2011, it was found that trade receivables included a bad debt of $650. It was decided to adjust the provision for doubtful debts to 4% of trade receivables. A debt of $420, which had been written off as bad in January 2010 was recovered in January 2011.

what was the effect of these events on the income statement for the year ended 31 May 2011?

A. Debit $384 B.Debit $410
C.Credit $430 D. Credit $456

A proper explanation would be appreciated.
Thanks and regards,
Bella Kath