Aluminum isn’t found free in nature, owing to its always ____ with other elements, most commonly with oxygen. 

_____ his position as mayor to give jobs to his friends，Williams is now under investigation．

Thank you for persuading my mother _______ the army.

Up till now, full use of this theory has been ________ in building large ocean-going ships.

On his way home, he found a wallet ____ on the ground, which he picked up and ____ in his schoolbag.