let f: $R\rightarrow R $ defined by $f(x) = min (|x|,x^2-1)$,then f is 

which one is correct ?

a) Differntiable everywhere except at one point

b) differntiable everywhere except at two points

i think option a ) is correct because f(x) = min$(|x|,x^2 -1)$= $|x|$ which is not diferentiable at 0

is it correct ???