The United States, where the most careful measurements have been done, discovered in 1982 that about one-fifth of its farmland was losing topsoil at a rate likely to diminish the soil’s productivity. Can “while” work here? No—not in this sentence structure. Here’s why: • “Where” refers to the place (the United States) and connects it to what was done there—measurements. • “While” introduces a time or contrast, but that doesn’t fit inside this clause. Short tip: Use “where” to describe place, especially when adding extra information with commas. Use “while” for contrast or time, like: While many countries ignore soil loss, the U.S. began measuring it.