OG8 阅读 Q48

Choice B is the best answer. The fourth paragraph explains that the Venus flytrap will close only if a second hair is stimulated to "push the calcium concentration over this threshold and spring the trap." But the last sentence of the paragraph notes that the calcium concentrations "dissipate over time," and if enough time elapses after the first trigger, "the final concentration after the second trigger won't be high enough to close the trap." It can be inferred, then, that if a large insect didn't touch a second trigger hair until after the calcium ion concentrations had diminished appreciably, the Venus flytrap would fail to close.
Choice A is incorrect because the fourth paragraph makes clear that if the calcium concentration goes above the trap's threshold, the plant will close, not remain open. Choice C is incorrect because as the third paragraph explains, the touching of the trigger hair and opening of the calcium ion channels don't act to keep the trap open but are instead a precondition for the closing of the trap (though closing will occur only if a second trigger hair is touched). Choice D is incorrect because the last sentence of the fifth paragraph explains that the threshold for the time that can elapse between the touching of the first and second trigger hairs is twenty seconds, meaning that a large insect touching two hairs within ten seconds would almost certainly make the plant close.