tvs are more useful than radios. from tv you can see and hear what is happening in the world. however, radios are not disappearing. they are still with us. and the number of listeners is becoming larger. do you know why?

　　one reason for this is the invention of the transistor. a transistor radio can be very small. it is very easy to be carried. besides, radio broadcasts are better for blind people. many old people don't have good eyesight. they can't watch tv, but can listen to music or news over the radio. what's more, a radio is cheaper than a tv set.