I chose D. I don't understand. If it hasn't gotten colder, then the sun hasn't set and the stars aren't out.

Here is the diagram of the passage:

SS --> Colder
SS --> SV

You know it has NOT gotten colder, as this is stated in the passage. So you have "~Colder".

The contrapositive of the first conditional is:
~Colder --> ~SS

So, since we know that it has not gotten colder, we know that the sun has not set.
Thus, answer choice "C" is correct.


'Why isn't "D" correct?':

We know that the temp has not dropped. But what about the stars? Well, we know:
SS --> SV

And we know that the sun has not yet set. This means we have:
~SS

But we CANNOT take these two pieces of info and conclude that the stars are not visible. To do so would require us to assume:
~SS --> ~SV

This is the INVERSE ERROR.