What two things must occur in order for a light ray to refract when it strikes the boundary of a different medium?
A. It must strike the boundary at 45 degrees, and its frequency must change.
B. It must strike the boundary at an angle, and its frequency must change.
C. It must strike the boundary at 45degrees, and its wavelength must change.
D. It must strike the boundary at an angle, and its speed must change

answer: the correct answer is   air pressure at the top of mount everest is higher than at the bottom of the grand canyon.

hope this : )

explanation:

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 =