Vanam
Point 1.
I feel ambiguity is not such a critical point in the given case. As such if you see, whether 'that' refers to mean temperature or simply the amount or even the same amount, it doesn't interfere with the meaning. So, there is no need to muse much about that. But, here 'the mean temperature' is the subject and 'the same amount' is the object. The first eligibility rests with the subject, failing which, perhaps we may consider the object.
Point 2.
Please change your idea that amount is plural. Look at this---
The amount of pension that a retired college teacher or scientist gets these days is phenomenal, running into lakhs of rupees per month.
The amount, in this case, is singular. Isn't it? The plural for 'amount' is 'amounts'.
I don't see the word temperatures in D. It is simply temperature. But even temperature when it refers to quantum is singular. For example, we generally say 'he had high temperature yesterday'.