Read the passage from a writer's draft. Paragraph 1: [1] A conservation group plans to ask hundreds of volunteers to record frog calls across a large region. [2] The volunteers can cover more places than a small research team, but a large number of observations does not guarantee that species are identified or recorded consistently. [3] ________ Paragraph 2: [4] In a pilot, brief audio training, a standard recording window, location checks, and expert review of uncertain clips greatly improved agreement among observers. [5] The project should build quality controls into participation from the beginning. Which sentence provides the needed warrant between the surrounding ideas about citizen-science frog surveys?