In Guericke's experiment to show the effect of atmospheric pressure,two copper hemispheres were tightly fitted to each other to form a hollow sphere and the air from the sphere was pumped out to create a vacuum inside. If the radius of each hemisphere is $R$ and the atmospheric pressure is $p$,then the minimum force required (when the two hemispheres are pulled apart by the same force) to separate the hemispheres is

• A
  $2 p \pi R^2$
• B
  $4 p \pi R^2$
• C
  $p \pi R^2$
• D
  $\frac{p}{2} \pi R^2$