Part A is a rational number, so adding another rational number (1/3) gives a rational result.
Part B is 2/3, so adding 1/3 gives a sum of 1, which is still a rational number.
Part C is 2*pi, which is irrational, and adding 1/3 gives a sum of 2*pi + 1/3, which still cannot be simplified into a rational expression.
Part D is an irrational number, and 1/3 - sqrt(17) also cannot be simplified into a rational number, so this is irrational.
Therefore the correct answers to this are C & D.