Most plants can make their own food from sunlight, (1)_____ some have discovered that stealing is an easier way to live, Thousan

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问题     Most plants can make their own food from sunlight, (1)_____ some have discovered that stealing is an easier way to live, Thousands of plant species get by (2)_____ photosynthesizing, and over 400 of these species seem to live by pilfering sugars from an underground (3)_____ of fungi(真菌). But in (4)_____ a handful of these plants has this modus operandi been traced to a relatively obscure fungus. To find out how (5)_____ are (6)_____, mycologist Martin Bidartondo of the University of California at Berkeley and his team looked in their roots. What they found were (7)_____ of a common type of fungus, so (8)_____ that it is found in nearly 70 percent of all plants. The presence of this common fungus in these plants not only (9)_____ at how they survive, says Bidartondo, but also suggests that many ordinary plants might prosper from a little looting, too.
    Plants have (10)_____ relations to get what they need to survive. Normal, (11)_____ plants can make their own carbohydrates through photosynthesis, but they still need minerals. Most plants have (12)_____ a symbiotic relationship with a (13)_____ network of what are called mycorrhizal fungi, which lies beneath the forest (14)_____. The fungi help green plants absorb minerals through their roots, and (15)_____, the plants normally (16)_____ the fungi with sugars, or carbon. With a number of plants sharing the same fungal web, it was perhaps (17)_____ that a few cheaters—dubbed epiparasites—would evolve to beat the system. (18)_____, these plants reversed the flow of carbon, (19)_____ it into their roots from the fungi (20)_____ releasing it as "payment".

选项 A、but
B、if
C、because
D、though

答案A

解析 语篇连贯。根据语篇,该句前后两个小句之间为转折关系,因此选but。
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