Jumping out of ________ airplane at ten thousand feet is quite ________ exciting experience．（NMET2002，26）

1. A.
   ／；the
2. B.
   ／；an
3. C.
   an；an
4. D.
   the；the

The research is so designed that once ____ nothing can be done to change it. (NMET2002)

A. begins B. having begun C. beginning D. begun