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21．While flying.a batmakes 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 【查看更多】

 

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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

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