Answer:
B. to determine the value of text/topic.
It is important to determine the central idea of a speech or essay because it helps the reader or listener to understand the main point or purpose of the text. The central idea is the most important point that the author is trying to convey to the reader or listener. Understanding the central idea can help the reader or listener to better understand the author's perspective, to evaluate the quality and relevance of the text, and to determine how the text relates to their own experiences or knowledge. While determining the central idea may help save time in some cases, this is not the primary reason why it is important.