These questions where you have to find the role or find the conclusion often present conclusions that don't begin with a conclusion indicator like thus or therefore. It is probably in your best interest to look at the argument as a whole and ask yourself what is the final point of the argument. Does the first sentence support the last or conversely, does the last sentence support the first.
You will find that it is actually the last sentence that supports the first, and considering it began with "thus", we know it is a subconclusion.
I hope this helps. Please let me know if you have any other questions.