In 1800, a group of adventurers set out on their exploring trip. Five days later, they came to _______ is now known as Quebec.

           A．what               B．which                 C．the place             D．where

---- Can you think of a case in which the ecology of an area is in balance?

  ---- Sure. You see, ______ an environment is undisturbed, the ecology of the area is in balance.

A．unless                        B．as                       C．though                D．where

_____ changed him was a mountain bike he received for his 15^th birthday.

A.What                        B. Whether            C. It                             D. That

The question _____ should do the work requires further discussion in the next meeting.

A. who                  B. that                   C. whom               D. why      

In 1969, _____ everyone had dreamed of happened the astronaut landed on the moon.

A. that                  B. what                C. which               D. as