Children don't understand initially what they are reciting, but________it will have an impact on their thinking.
句意：孩子们不知道他们背的是什么，但它会渐渐地影响他们的思维。 casually 意为“随意地”， especially 意为“尤其地"， regularly 意为“按时地”， gradually 意为“渐渐 地”。根据句意，故选 D。