The author mentions cereal packets to illustrate that ( ).

参考解析：

定位第三段，Writing, visualizing and prototyping can stimulate the flow of new ideas. Cialdini cites scores of research papers and historical events that prove that even something as simple as writing deepens every individual s engagement in the project. It is, he says, the reason why all those competitions on breakfast cereal packets encouraged us to write in saying, in no more than 10 words: 写作.可视化和原型设计可以激发源源不断的新想法。Cialdini引用了大量的研究论文和历史事件来证明即使是像写作这样简单的事情也能加深每个人对这个项目的投入。他说，这就是为什么所有这些包装在早餐谷物包装上的竞赛鼓励我们写下不超过10个字的内容。可知第一句是观点句，下面的内容都是论据，来证明观点的。A项：简单的事情有时能帮助你拥有更深刻的信念；B项：深化人民对一项任务的参与是非常容易的；C项：写作是激发新想法的最有效方式；D项：顾客倾向于相信简短的话。故本题正确答案选C。