How are the eggshells prepared?
细节题。根据Here is how it’s done．You should save your eggshells and rinse them in water，then cook them lightly on the baking pan in the oven；set at a low temperature until they are dry but not brown．可知D正确，A选项They are dried in the sun与原文中set at a low temperature until they are dry but not brown不符；B选项They are soaked in water，原文中用的是rinse“冲洗”，而非soak“浸泡”；C选项They are cooled in the refrigerator原文未提到。因此，正确答案是D。