Question

Humans can hear sounds with frequencies up to about 20.0 kHz, but dogs can hear frequencies up to about 40.0 kHz. Dog whistles are made to emit sounds that dogs can hear but humans cannot. If the part of a dog whistle that actually produces the high frequency is made of a tube open at both ends, what is the longest possible length for the tube? (Assume a temperature of 20° C.)

I got 0.43 cm for this question, but answer has to be in mm, so i put 4.3mm but its still wrong. I keep trying its giving me wrong. Could someone please help me out. Thank you.