Apolice siren sounds different as it comes close to you than when it moves away from you. what term explains how this happens?
a. diffraction
b. the doppler effect
c. rarefaction
d. shockwaves

b. the Doppler effect

Explanation:

Doppler effect describes the apparent change in the frequency heard by the listener than the frequency emitted by the source when there is relative motion between the two. When the source and listener approach each other, the apparent frequency is higher than the source and when they move away, the apparent frequency heard by the listener is lower than the source.

Due to Doppler's effect, a police siren sounds different when it comes closer than when it moves away. When it is closer, higher pitch can be heard than when it is moving past away.

i guess it’s a and b

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =