Read the following passage, then answer the question.

When Marcus first moved to the city, the constant noise of traffic kept him awake for weeks. He bought thick curtains, then earplugs, then a small machine that hummed a steady wash of white sound, but nothing seemed to help. Then one evening, exhausted after a long shift, he fell asleep before he remembered to be bothered. After that, the sounds outside his window slowly faded into the background of his life. He could not say exactly when it happened, only that one morning he realized he no longer heard the traffic at all.