--Who is the old man talking with your teacher? ---I don’t know. I him before.
Once ___ what to do, these computers will show great power in carrying it out.
He talked about the classmates and the school ___ he had visited.
Americans eat __ vegetables per person today as they did in 1910.
I wonder if you know it ___ .
The number of people invited ___ fifty, but a few of them ___ absent for different reasons.