1. Given two numbers, n and k ( 0< n, k <=12), generate all the pemutations taking k letters from n letters (nPk) considering first n small alphabets.
2. Use the idea of backtracking (recursion).

Scientists have changed the model of the atom as they have gathered new evidence. one of the atomic models is shown below. what experimental evidence led to the development of this atomic model from the one before it? a few of the positive particles aimed at a gold foil seemed to bounce back. the colors of light emitted from heated atoms had very specific energies. experiments with water vapor showed that elements combine in specific proportions. cathode rays were bent in the same way whenever a magnet was brought near them.