According to the passage, which of the following types of nectar-feeding pollinators is likely to be an unreliable pollinator of a particular cactus flower?

Line	In the Sonoran Desert of northwestern Mexico and
	southern Arizona, the flowers of several species of
	columnar cacti—cardon, saguaro, and organ
	pipe—were once exclusively pollinated at night by
(5)	nectar-feeding bats, as their close relatives in arid
	tropical regions of southern Mexico still are. In these
	tropical regions, diurnal (daytime) visitors to columnar
	cactus flowers are ineffective pollinators because,
	by sunrise, the flowers’ stigmas become unreceptive
(10)	or the flowers close. Yet the flowers of the Sonoran
	Desert cacti have evolved to remain open after sunrise,
	allowing pollination by such diurnal visitors as bees and
	birds. Why have these cacti expanded their range of
	pollinators by remaining open and receptive in daylight?
(15)	This development at the northernmost range of
	columnar cacti may be due to a yearly variation in the
	abundance—and hence the reliability—of migratory
	nectar-feeding bats. Pollinators can be unreliable
	for several reasons. They can be dietary generalists
(20)	whose fidelity to a particular species depends on
	the availability of alternative food sources. Or, they
	can be dietary specialists, but their abundance may
	vary widely from year to year, resulting in variable
	pollination of their preferred food species. Finally, they
(25)	may be dietary specialists, but their abundance may
	be chronically low relative to the availability of flowers.
	Recent data reveals that during spring in the
	Sonoran Desert, the nectar-feeding bats are
	specialists feeding on cardon, saguaro, and
(30)	organpipe flowers. However, whereas cactus-flower
	abundance tends to be high during spring, bat
	population densities tend to be low except near
	maternity roosts. Moreover, in spring, diurnal cactus-
	pollinating birds are significantly more abundant in
(35)	this region than are the nocturnal bats. Thus, with bats
	being unreliable cactus-flower pollinators, and daytime
	pollinators more abundant and therefore more reliable,
	selection favors the cactus flowers with traits that
	increase their range of pollinators. While data suggest
(40)	that population densities of nectar-feeding bats are
	also low in tropical areas of southern Mexico, where
	bats are the exclusive pollinators of many species
	of columnar cacti, cactus-flower density and bat
	population density appear to be much more evenly
(45)	balanced there: compared with the Sonoran Desert’s
	cardon and saguaro, columnar cacti in southern Mexico
	produce far fewer flowers per night. Accordingly,
	despite their low population density, bats are able to
	pollinate nearly 100 percent of the available flowers.

A、A dietary specialist whose abundance is typically high in relation to that of the flower
B、A dietary specialist whose abundance is at times significantly lower than that of the flower
C、A dietary generalist for whom that flower’s nectar is not a preferred food but is the most consistently available food
D、A dietary generalist for whom that flower’s nectar is slightly preferred to other available foods
E、A dietary generalist that evolved from a species of dietary specialists

答案：B

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问题：请教张慧雯老师，本题我的定位点和思路：the nectar-feeding bats are specialists feeding on cardon, saguaro, and organpipe flowers. However, whereas cactus-flower abundance tends to be high during spring, bat population densities tend to be low except near maternity roosts.以及 organ pipe—were once exclusively pollinated at night by nectar-feeding bats, as their close relatives in arid tropical regions of southern Mexico still are.然后就在ab里面纠结，请老师指出我的问题。

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