Children don't understand initially what they are reciting, but it will have an impact on their thinking.
casually
especially
regularly
gradually
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。