I would have come sooner but I ________ that they were waiting.
— People should spare no efforts to protect the environment. — ________. Air pollution does harm to our health.
The _______ of students in our class are from the country; only five are from cities.
What would you give her _______ the painting she collected?
We were swimming in the lake _______ suddenly it began to rain heavily.
Don’t try to cheat in the exam; you will never _______ it.