请帮忙翻译一下，谢谢3

Client Needs and PrioritiesThe PSA and the TV ad are works commissioned by a client. The client needs a solution to a communication problem that the writer must provide. We alluded to this discipline of the professional writer in Chapters 3 and 4. You write for someone or rather someone who represents the interests of an organization or a corporation. Later we will look more closely at another kind of writing for a producer of entertainment films or programs. The entertainment script is different from commissioned works because neither the producer nor the writer can know for sure what a good script is until it is produced, shown to an audience, and validated by box office or audience ratings. Commissioned programming doesn’t have an audience measurement expressed in terms of box office revenue. Successful communication can only be measured by quantifying audience responses as changes in sales or behavior.Advertisers expect to measure the effect of an ad in increased sales. Otherwise,there is no business sense in spending money on it. A PSA often aims to change people’s behavior. It is much more difficult to garner information that positively proves the effectiveness of the PSA. Changes in behavior are much more difficult to achieve than changes in the buying choices of the public.Writing for clients is often challenging and exciting precisely because you have a problem, know the desired result, and have to devise a solution. The seven-step method of Chapter 4 is an excellent way to approach these assignments. The process of analysis is really important for writing PSAs. Although you do not now have a client, you must practice writing as if you had a client to satisfy. Your creative ideas must do the job. One of the constraints of this kind of writing is that the length is fixed by the client. Because the resulting product is transmitted in commercial breaks, its length must be exact to the second, as that is how airtime is bought and sold.