The same amount of cucumber seedlings were planted in the two experimental greenhouses. Magnesium salt was applied in the first greenhouse, but the second was not added. The first produced 10 kg of cucumber and the second produced 5 kg. Nothing was applied to the greenhouse other than water, and the higher yields of the first greenhouse must be due to magnesium salts. Which of the following, if true, has most severely weakened the above argument? ? ['A small amount of magnesium salt is found in the soil of the two experimental greenhouses.', 'The third experimental greenhouse applied a high nitrogen fertilizer but no magnesium salt, yielding 7 kg of cucumber.', ' different cucumber varieties planted in both experimental greenhouses.', 'The soil quality and sunshine amount of the two experimental greenhouses are different.']