问答题 Once the pollen has been snatched by the wind, the fate of the pollen is obviously up to the vagaries of the wind, but not everything is left to chance. [A] Wind-borne pollen is dry, rounded, smooth, and generally smaller than that of insect-pollinated plants. [B] Small grains may be blown farther but they are also more prone to be whisked past the waiting stigma because smaller particles tend to stay-trapped in the fast-moving air that flows around the stigma. [C] But stigmas create turbulence, which slows the air speed around them and may help pollen stick to them. [D] Look at the four squares [■] that indicate where the following sentence could be added to the passage. But size is a two-edged sword. Where would the sentence best fit?