The pressure _____ causes Americans to be energetic, but it also puts them under a constant emotional strain (过度紧张).

1. A.
   to compete
2. B.
   competing
3. C.
   to be competed
4. D.
   having competed

The pressure _____ causes Americans to be energetic, but it also puts them under a constant emotional strain (过度紧张).

A. to compete              B. competing

C. to be competed                D. having competed