Millions of people watched that first moon landing on television, their hearts in their mouths, aware of how difficult and dangerous an adventure it was, and what risks had to be ____.

1. A.
   taken
2. B.
   take
3. C.
   to take
4. D.
   taking

 Millions of people watched that first moon landing on television, their hearts in their mouths, aware of how difficult and dangerous an adventure it was, and what risks had to be ____.

   A. taken                  B. take                  C. to take              D. taking

Millions of people watched that first moon landing on television, their hearts in their mouths, aware of how difficult and dangerous an adventure it was, and what risks had to be ____.

 Millions of people watched that first moon landing on television, their hearts in their mouths, aware of how difficult and dangerous an adventure it was, and what risks had to be ____.

   A. taken                  B. take                  C. to take              D. taking