As is known to all language learners, the newly learnt words will soon be forgotten unless ______ used in everyday communication.

Leave your key with a neighbor         you lock yourself out one day.

We often say that something is “as light as air”, but air is＿＿＿light. 

When they knew there was a panda in the zoo, the children could _________ wait to see it.

Tourism,    it brings in money to a country,can also result in damage to the World Heritage sites there.