When the school bus arrived at the zoo, the bus door ______ open and all the children rushed out.

After his father died, the boy was _______ by his devoted mother, who attached importance to his education. 

You should get your ideas ________ when you are putting _______ an ad campaign.

The most serious problem we are facing now is how to ______ all the pollution. 

----How often do you _________ the dictionary when you are doing reading? ----Not very often. Only when I find it is necessary.