It is pretty well understood_______ controls the flow of carbon dioxide in and out the atmosphere today.

When it _____ air pollution, Beijing faces the 3 “C” s: cars, coal and construction, which lead to Beijing’sserious air pollution.

_________ the final exams, I would have gone to the concert yesterday

“ Blood Moon” appeared in the sky between 6:24 pm and 7:24 pm on October 7,2014, which is quite unique for moon.

The magazine was started by Becky,______ I know.