Two experimental garden plots were each planted with the same number of tomato plants. Magnesium salts were added to the first plot but not to the second. The first plot produced 20 pounds of tomatoes and the second plot produced 10 pounds. Since nothing else but water was added to either plot, the higher yields in the first plot must have been due to the magnesium salts.
Which of the following, if true, most seriously weakens the argument above?
(A) A small amount of the magnesium salts from the first plot leached into the second plot.
(B) Tomato plants in a third experimental plot, to which a high-nitrogen fertilizer was added, but no magnesium salts, produced 15 pounds of tomatoes.
(C) Four different types of tomatoes were grown in equal proportions in each of the plots.
(D) Some weeds that compete with tomatoes cannot tolerate high amounts of magnesium salts in the soil.
(E) The two experimental plots differed from each other with respect to soil texture and exposure to sunlight.
A. Doesn't weaken the argument. It tries to confuse you making you think that if some salt leached then we would have similar results. But since a small amount leached it's not relevant enough.
B. Doesn't weaken the argument, just adds a different variable.
C. If they were grown in equal proportions then it strengthens the argument instead of weakening it.
D. It strengthens the argument: some weeds that compete with tomatoes died in the 1st plot while they may have grown in the second, affecting the yield of the second plot.
E. If true, it really weakens the argument, showing that we couldn't compare the two plots as equal, allowing other variables to play some effect on the yields.