Every possible means _____ to prevent the air pollution, but the sky is still not clear.

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is used are used has been used have been used

has been tried tried is being tried has tried

meet with content with cater for satisfy with

has tried has been tried was tried were tried

have been tried; by this means has been tried; in this way w m has been tried; in this means are tried; by this means

hunting for carrying on looking after bringing up

have tried have been tried has tried has been tried

have; polluting has; polluted has; from being polluted have; being polluted

have，in have，by has，in has，by

has tried has been tried is being tried tried

has tried has been tried was tried were tried

have been tried; by this means has been tried; in this way has been tried; in this means are tried; by this means

is used are used has been used have been used

is used are used has been used have been used

meet with content with cater for satisfy with

has tried; has proved tried; proves has been tried; proves is being tried; is proved

was tried were tried has been tried have been tried

is used are used has been used have been used

was tried were tried has been tried have been tried