Consider an element with finite dimensions. In general which among the following equations is correct for change in energy of element during a time span dt?

• A. [Heat generated in the element during time dt] + [Heat flow into the element during time dt] + [Heat flow out of the element during time dt]
• B. [Heat generated in the element during time dt] + [Heat flow into the element during time dt] – [Heat flow out of the element during time dt]
• C. [Heat generated in the element during time dt] – [Heat flow into the element during time dt] – [Heat flow out of the element during time dt]
• D. none of the above