4. People begin to know that most environmental problems exist because necessary measures for preventing them _______taken in the past.
People are puzzled _____ they read the book.
We have done things we ought not to have done and_____ undone things we ought to have done.
When you write to him, please _____ him.
-- Tom works hard at English.?-- _____, and _____.
After he gave a report about the school, Mr White____ the visitors around it.