Fill in the blank with the correct form of the verb.
1. Many people in the world don’t have a decent place to live. To help solve this problem, more low-cost housing needs __________ (build).
2. Disease is still a problem in some countries. More vaccines should __________ (produce) so that everyone can live a healthy life.
3. Millions of stray animals are brought to animal shelters every year. A nationwide campaign should __________ (undertake) to encourage people to adopt these pets.
4. We have a lot of street crime in our city. More police officers have __________ (train) to patrol the community on foot.