The hemispheres of Magdeburg...

To demonstrate the force of air pressure, Otto von Guericke had manufactured two hemispheres that form a hallow sphere when put together. With the help of a piston pump, which he had invented himself, he removed all the air from the hallow sphere. After that, eight horses were yoked to each hemisphere that should pull on them in opposite directions. The 16 horses did not manage to separate the hemispheres. They could only be separated after Otto von Guericke let air back into the sphere.

Why couldn't the horses manage to separate the hemispheres? Which of the utterances below explains the process? (Click on the correct answer)

With removing the air from inside of the sphere...

a) the pressure outside of the sphere was much higher than inside of it. Therefore, the two hemispheres were pressed towards each other.

b) a vacuum arouse inside of the sphere. This vacuum created a huge suction effect so that the two hemispheres could not be separated, not even with 16 horses

c) the horses were weakened, so that they did not have enough force to separate the two hemispheres.