Because bats produce high frequency ultrasound waves which cannot be heard by human ear these waves strike on the insects (prey) and reflect back as echo which is heard by the bats on hearing the bat moves towards the prey and catches it 
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Bats catch their prey at nt by the method of echolocation
bats emit high frequency ultrasound squeaks from the object  while flying and listen to the echoes produce by the reflection of their squeak fm the object  in their path fm the time aken to echoes to be heard bats can judge the distance of the object intheir path and hence they catch their prey
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plzzz mrk it as best
plzz mark this and prev ques as best