根据句意，用所给单词的适当形式填空。
1．You mustn't be a ________ (care) driver.
2．The girl is old enough to look after ________ (she)．
3．She is really _____________ (understand), and everyone likes her.
4．It's five ___________ (mile) from my home to school.
5．After long work, I _____________ (certain) felt tired.
1.careless 2.herself 3.understanding 4.miles 5.certainly