As a child I used to wash my parents' car to earn some _____ mon

As a child I used to wash my parents' car to earn some _____ money.
A、paper
B、easy
C、private
D、difficult
【正确答案】：C
【题目解析】：paper纸；easy容易的；private私人的；difficult困难的句意：当我还是个孩子的时候，我经常给父母洗车来挣些私房钱。