Clarify statement and plan of attack

As in (a) I think a hint should clarify what has to be done if we think the statement is true, and what has to be done if we think the statement is false.

Then we have to recall continuity and differentiability. The latter is done in the solution, I think this should be a hint. Also, the two conditions for non-differentiability given in the solution are not the only things that can go wrong. So if we want to be on the safe side add let's add something like examples what can go wrong instead of not differentiable if, to not have people confuse if with only if or if and only if.

Also stress that one example would be enough to solve the question. Although this will be clear once the hints are fixed.