A BRIDGE HAS BEEN BUILT ACROSS THE RIVER. THIS IS VERY CONVENIENT....

12. A bridge has been built across the river. This is very convenient.

A. Once a bridge has been built across the river, it is very convenient.

B. A bridge has been built across the river, which is very convenient.

C. A bridge has been built across the river which is very convenient.

D. A bridge has been built across the river, so this is very convenient.