I.冠词填空：在下列句中空白处填入适当冠词,不需用冠词处划
1．There is _______ picture of _______ elephant on _______ wall．
2．This is _______ useful book．I've read it for _______ hour．
3． _______ elephant is much heavier than _______ horse．
4． _______ doctor told him to take _______ medicine three times _______ day．
5．Let's go out for _______ walk．