Why does the sound of something moving away from you seem to change to a lower and lower pitch?
a)
as the source moves away, the increased distance creates more interference; so the pitch drops.
b)
as the source moves away, the frequency of the sound waves is being compressed, so the pitch drops.
c)
as the source moves away, the sound waves speed up to reach the observer; this causes the pitch to drop.
d)
as the source moves away, the sound waves stretches out relative to the person standing still; so the pitch drops.

answer: - b cells

explanation: -

there are many types of white blood cells present in our body. one such class of white blood cells are the lymphocytes. the b cells belong to the lymphocyte class of wbcs. the b cells perform their action against the pathogen by producing specialised proteins called as antibodies. these antibodies are produced as and when the b cells encounter pathogens in response to the antigens present on the surface of the pathogen. these antibodies bind to the antigens present on the pathogen and then, target them for destruction by other cells of the immune system.

so, the b cells mediate their action by identifying the antigens present on the surface of the pathogens and then producing antibodies in return to mediate its effect.