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!