Teachers recommend parents ______their children under 12 to ride bicycles to school for safety.

I have tried to solve the problem, but it is so difficult that it ______ not only me but also my friend. [ ]

“Is Jack on duty today?”“It ____ be him. It’s his turn tomorrow.”

_________the fact that his first experiments had failed, ProfSmith persisted in his research

No one had told Smith about ________ a lecture the following day.