Use your brain power:
If the ruler is plucked while it is held with 25 cm of it off the table, does it make any sound? If there is no sound, look for the reason why it is so.

Answer

If the ruler is held with 25 cm of it off the table land is plucked, it does not produces any sound; because the frequency of vibrations produced that time would be very less. We can only hear sounds of particular frequency.