Which of the following describes a condition in which an individual would not hear an echo?

a. an individual is standing 30 meters in front of the reflecting surface.

b. the reflecting source is approximately 21.5 meters from the individual.

c. an individual is standing in the range of the sound waves coming from the original sound source as well as the reflected waves.
d. a sound wave reaches the ear and the reflected wave reaches the ear less than 0.1 second later.

Answer asap need by wednesday morning explain how a buffer works, using an ethanoic acid/sodium ethanoate system including how the system resists changes in ph upon addition of a small amount of base and upon addition of a small amount of acid respectively. include the following calculations in your i. calculate the ph of a solution made by mixing 25cm3 0.1m ch3cooh and 40cm3 0.1m ch3coo-na+. [ka = 1.74 x 10-5 m] ii. calculate the ph following the addition of a 10cm3 portion of 0.08 m naoh to 500cm3 of this buffer solution. iii. calculate the ph following the addition of a 10cm3 portion of 0.08 m hcl to 200cm3 of the original buffer solution.