Reading Comprehension
试题详情
文章：
| 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. |
题目：
According to the passage, present-day columnar cacti in the Sonoran Desert differ from their close relatives in southern Mexico in that the Sonoran cacti
选项:
A、being converted to cooperative ownership is if to sign
B、being converted to cooperative ownership is whether they should be signing
C、being converted to cooperative ownership is whether or not they sign
D、conversion to cooperative ownership is if to sign
E、conversion to cooperative ownership is whether to sign
答案:
E
