---It’s a long time _____ I saw you last.---Yes, and what a pity it is now that it will be a long time _______ we see each other again.
It was not until all the milu deer ________ that we realized we should build a natural park to protect them.
Do you remember a certain occasion ________ you were in trouble and at that moment I gave you a hand.
You may catch up with others__________you work a little harder.
__________ the students are doing their maths exercises with the help of the teacher.
---- ______was it______ you discovered the secret of his? -----Totally by chance .