The telephone wires that lead from your home to the homes of friend, relatives, and even strangers are all covered by a thick, protective tube that insulates them from damage. This tube helps eliminate damage from things like water, birds feet, bugs, dirt, weather, tree branches, and other potential interruptions in communication. In the human neuron, the same sort of protection is provided by the .

Apolitical machine ~ apex 

Ifound the exercise on the internet. attached is the list mentioned above.in order for a compound to be considered optically active it can't have any axis or plan of symmetry. the molecule  shouldn't also  be superimposable on its mirror image.with such considerations, the compounds that are optically active are: -third compound on the first image is optically active; -second and third compound from the second image are optically actives.