Read the following paragraph and underline the topic sentences.
New technology has made possible the development of miniature cassette recorders. The Sony Walkman, for instance, is small enough to fit into a pocket or handbag. It has become the constant companion of millions of Americans who walk, shop, jog, and commute to the music of their choice. The wide availability Walkman and its moderate price have made it so popular that it can be found in many public places. The muffled sound of music, a smiling face, and tapping feel reveal the Walkman's presence. There is no use shouting hello or good-bye at Walkman users because they won't hear you. They are in a musical world of their own.