'I. the polynomial must cut the x-axis at least once' is correct if we consider x as real. So 'II. if P(alpha) = 0, then P(conj. alpha)=0' is not correct.
II is correct if we consider x as complex. So I is not correct.
.: the statements are mutually exclusive.
simple and easy to understand answer by considering energy:
more fuel, greater height reached, greater kinetic energy when the rocket falls back to earth, .: greater speed.
This method of finding the rational roots works only when they exist. It works for most of the questions in textbooks because the questions were prepared for it to work, i.e. rational roots were used to construct the polynomials. Just think about this: does a polynomial always cross the x-axis...