The unit used to measure the intensity of sound is the decibel (dB). It is a logarithmic unit used to express the ratio of a physical quantity, such as power or intensity, relative to a specified or implied reference level. The decibel is commonly used in acoustics and audio technology.
- Option (A) "Particles per hundred" is incorrect as sound is not measured in terms of particles.
- Option (C) "Particles per thousand" is also incorrect for sound measurement.
- Option (D) "Nanometer" is incorrect as nanometers are used to measure length, not sound.
Thus, the correct answer is (B) Decibel.

A ladder of fixed length \( h \) is to be placed along the wall such that it is free to move along the height of the wall.
Based upon the above information, answer the following questions:
(iii) (b) If the foot of the ladder, whose length is 5 m, is being pulled towards the wall such that the rate of decrease of distance \( y \) is \( 2 \, \text{m/s} \), then at what rate is the height on the wall \( x \) increasing when the foot of the ladder is 3 m away from the wall?