While flying, a bat(蝙蝠)makes a sound which hits things and comes back to its ears．In this way it’s able to know ________ the things are and avoid bumping into them．

1. A.
   how
2. B.
   where
3. C.
   what
4. D.
   which

While flying，a bat（蝙蝠）makes a sound which hits things and comes back to its ears．In this way it's able to know           the things are and avoid bumping into them．

A．how                    B．where                  C．what                      D．which