[单选题]Read the following paragraph and choose the topic sentence.At one time, transistor radios were not practical, because they were too expensive, Now all of that has changed. With the reduced price of transistors and the cheaper costs of mass production, the transistor radio is cheaper than the old-style tube model. In addition, transistor radios do not heat up like the old tube radios, so they will not wear out as quickly. Also, transistor radios can be made much smaller because transistors are smaller than tubes. Furthermore, transistor radios are more reliable. They have fewer parts, so less can go wrong.