Ram-soaked soil contains less oxygen than does drier soil.The roots of melon plants perform less efficiently under the lowoxygen conditions present in rain-soaked soil. When the efficiency of melon roots is impaired, the roots do not supply sufficient amounts of the proper nutrients for the plants to perform photosynthesis at their usual levels. It follows that melon plants have a lower-than-usual rate of photosynthesis when their roots are in rain-soaked soil. When the photosynthesis of the plants slows, sugar stored in the fruits is drawn off to supply the plants with energy. Therefore, ripe melons harvested after a prolonged period of heavy rain should be less sweet than other ripe melons.
In the argument given, the two highlighted portions play which of the following roles?
答案：C