The hemisphere of Magdeburg

To demonstrate the power of air pressure Otto von Guericke had two hemishpreres manufactured which put together made a hollow ball. Air was sucked out of the hollow ball using a special vacuum pump, he invented. Afterwards on each hemisphere 8 horses were harnessed trying to pull them apart.The 16 horses didn´t suceed in pulling them apart. The hemispheres could only be separated when Guericke allowed the air to flow back into them.

Why weren´t the horses able to separate the hemispheres? Which of the following statements listed below explain this action (hit the right answer) explain
Because air was sucked out of the inside of the hemisphere...

a) Was the external air pressure on the outside of the spherical shell that much bigger than at the inside of it and therefore pressed both spherical shells tightly together.

b) Did a vacuum develop at the inside of the sphere that caused this tremendous sucktion effect preventing 16 horses from pulling it apart.

c) Had the horses been weakened so much they weren´t able to exert enough force to separate both hemispheres.