Answer: Option A) a sound wave encounters an object that is larger than the wave length of the sound
Explanation:
An acoustic shadow is formed when a sound wave of low wavelength (higher frequency) is unable to diffract (bend around) large physical objects such as building structures. This lessens the rate of travel of sound, and low sound is heard on the other side.
Well a group of organs that work together is called a system. For example the digestive system (stomach, liver, esophagus, small intestine, large intestine, gall bladder), also the endocrine system... :)