cash book

Hallo, In the following example: A debit entry in the cash book will have which effect on the level of a bank overdraft and a bank balance? Bank overdraft Bank balance A Increase Increase B Decrease Decrease C Increase Decrease D Decrease Increase I answered D, but the book's answer is C, why am I wrong, this is money we've received, so B o/d, should be less, and Bank bal should be more, as we have more money. Thank you!