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．(E) The two experimental plots differed from each other with respect to soil texture and exposure to sunlight.