1) For the last six months I have been working on the thesis.
2)For the last six months I have worked on the thesis.
I know that first sentence is correct. It means I started working six months back and i am working even now in the sixth month.
But my book is saying second sentence is wrong.Can't we use second to mean _ For the last six months (till now six months are completed), I worked on the thesis (I have just stopped working recently) ?