求教两道prep我觉得很难理解的题目

17.   (27669-!-item-!-188;#058&003302)

Urban air contains more sulfur dioxide than does rural air, and plants in cities typically grow more slowly than do plants in rural areas.  In an experiment to see how much of the difference in growth is due to sulfur dioxide, classes in an urban and a rural school grew plants in greenhouses at their schools and filtered the greenhouse air to eliminate sulfur dioxide.  Plants in the urban greenhouse grew more slowly than those in the rural greenhouse.

Which of the following, if true, would it be most important to take into account in evaluating the result?

(D) Heavy vehicular traffic such as is found in cities constantly deposits grime on greenhouse windows, reducing the amount of light that reaches the plants inside.

我选B,但给出的答案是D,我想不通啊！

State spokesperson:  Many businesspeople who have not been to our state believe that we have an inadequate road system.  Those people are mistaken, as is obvious from the fact that in each of the past six years, our state has spent more money per mile on road improvements than any other state.

Which of the following, if true, most seriously undermines the reasoning in the spokesperson's argument?

(A) In the spokesperson's state, spending on road improvements has been increasing more slowly over the past six years than it has in several other states.

（E) Only states with seriously inadequate road systems need to spend large amounts of money on road improvements.

此题我选A,但答案是E,无法理解，求教高人指点．