Explanation

1) The passage mentions that ragwort’s impact on New Zealand’s agriculture was especially severe because the plant’s proliferation “coincided with sweeping changes in agriculture that saw a massive shift from sheep farming to dairying.” The severity of the impact was increased because cattle, which were displacing sheep, are much more sensitive than sheep to the toxins contained in ragwort.
This points to Choice E as the correct answer choice. Nothing in the passage suggests that the proliferation of ragwort coincided with a decline in agriculture (Choice A), occurred faster than it might have done (Choice B), or made the plants more toxic (Choice C). There is a suggestion that ragwort honey might not be safe for humans, but there is no indication that this made the timing of the proliferation particularly unfortunate.

2) Choice B is correct. The question asks about the problems ragwort poses to dairy farmers.
Choice A is incorrect: The passage does not mention the effect of ragwort consumption on the milk produced by cows.
Choice B is correct: The passage mentions that livestock will eat ragwort “once it displaces grass and clover in their pasture.” Choice C is incorrect: The passage claims that “livestock generally avoid grazing where ragwort is growing,” but does not make a distinction between cattle and sheep.