What does the president mean by the simile "like closing the barn door after the horses are out"?

a.
the disease has already entered the country, so closing the border won't keep it out.
b.
u. s. border crossings usually are housed in big buildings referred to as "barns."
c.
the president used the simile because he knows that border gate is similar to a barn door.
d.
although called "swine flu," the president knows that the disease is spread by horses.