Which number can replace [[$ x $]]?

a) [[$ \sqrt {x} $]] = [[$ 2 $]]

[[$ x $]] =

b) [[$ \sqrt {49} $]] = [[$ x $]]

[[$ x $]] =

c) [[$ \sqrt {x} $]] = [[$ 12 $]]

[[$ x $]] =

d) [[$ \sqrt {0,16} $]] = [[$ x $]]

[[$ x $]] =

e) [[$ \sqrt {x} $]] = [[$ 100 $]]

[[$ x $]] =