将下列主动语态变成被动语态。
1. We must finish the work in two days.
The work must _____ in two days.
2. Mrs. Green has given Robert a present for Christmas.
Robert _____ a present for Christmas.
3. We have used paper and printing for ages.
Paper and printing _____ for ages.
4. He doesn't think computers can replace books.
He doesn't think books _____ by computers.
5. We have bought some gifts for the poor children.
Some gifts _____ for the poor children.