Complete the sentences with the correct passive form of the verbs.

1. it's obvious that more research into the new medicine (ought to / to do) before it was made available.
2. the results of the experiment (must / to check) before they were shown.
3. not all health advice (may / to understand) in the past.
4. there's a possibility that we (might / to lie) to by the drinks industry about how much water we need.