It’s 8 o’clock. The students _________ an English class.
We are wondering ________ we can prevent people from polluting the earth.
The gravity on Mars is only about ________ of that on Earth.
It is hard to imagine what life __________ in 100 years.
My friend John insisted that he had seen ______ alien the day before.
--- What pleasant weather! Shall we go boating on the lake?--- _______.