While B is the best answer, in truth native speakers of English will say any of A, B, C, or D, with B and C being the most common, followed by A and D. "Gives off" means to generate, whereas "gives out" could also mean to "collapse," "fail to function," or "break down." So I would choose "give off" over "give out" in this circumstance. Furthermore, while it is true that "heat" is uncountable, and we would not say "I felt a heat," when we add an adjective before the uncountable noun, we can often allow the use of an indefinite article, such as "the fire gave off *a* good heat." After all, I am sure you have heard/read English speakers say/write "I saw a bright light." Light is also uncountable, but you can say either "I saw a bright light coming from the sky" OR "I saw bright light coming from the sky."

呃，不好意思还要打扰你下了，我问了下英语老师呢，其实狭义一点还是give out比较好，give out 有直接表意思是散热器发热。give off 比give out 更加广义，give off 可以随时代替give out。http://zhidao.baidu.com/question/294616512.html看看这个吧。这语法是很搞的呢。

是嗎? 原來是可以互通. 謝謝指教! :)

其实口语的话两个是可以通用的。做题目要咬文嚼字一点的确是b对，原因我也解释不了，这两个没有原则上的差别。