Choose all the statements below that are TRUE.

a)If a continuous function has a vertical tangent line at a point x in its domain, then it is not differentiable at that point.

b)By definition, if lim xa f'(x) exists, then f' is differentiable at a and the limit is f'(a).

c)If a function is not continuous at a point x, then it is definitely not differentiable there.

d)If a function is continuous at a point x, then it must be differentiable there.

e)If a function is differentiable at a point x, then it must be continuous there.

f)If a function is not differentiable at a point x, then it is definitely not continuous there.