Some English scientists are identifying problems ____ technology in the 21st century. If solved, the solutions could change the world.

1. A.
   facing
2. B.
   faced
3. C.
   being faced
4. D.
   having faced

 Some English scientists are identifying problems ____ technology in the 21st century. If solved, the solutions could change the world.                         

       A. facing B. faced  C. being faced D. having faced

Some English scientists are identifying problems ____ technology in the 21^st century. If solved, the solutions could change the world.

A．facing                         B．faced                  C．being faced          D．having faced