Which of the following is closest in meaning to the underlined expression “lagging behind”?A. hide behindB. run behindC. walk behindD. fall behind

此题为词汇理解题。根据句子“Today, when children are exposed…parents often find themselves lagging behind in computer skills.”可知，lag behind意为“落后于”。四个选项中，只有fall behind表示“落后”的意思。