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e is probably the best answer for an introductory course. However, a is the best answer overall.
consider d) Bees are not the only vector for pollen, trees are usually pollinated by wind, as are grasses
b) is silly, pollen is in great abundance. Have you ever parked under a pine tree?
d) Bees go from flower to flower, for nectar or pollen. I am not certain fidelity to flowers is evolutionary important to plants, as most are not pollinated by bees.