Final answer:
The question cannot be answered with the provided options and reference information as they do not directly relate to the average speeds of speaking and writing.
Step-by-step explanation:
The question asks for the speed differential between speaking and writing. To find the speed differential, we would need to know the average speeds of speaking and writing and then calculate the difference. However, the provided options and reference information do not contain any data about the average speeds of speaking and writing, nor do they provide a context in which to calculate such a differential. Without additional context or information, it is not possible to determine the speed differential between speaking and writing from the given options.