She was just about to explain _________ she hadn't passed her maths test chiefly out of careless

[单选]

She was just about to explain _________ she hadn't passed her maths test chiefly out of carelessness _________ her uncle came.

• A、her mother；when

• B、to her mother why；that

• C、her mother that；when

• D、to her mother that；when