I also advocate coming up with a potentially correct assumption before looking at the answers. If you get good at this, you can save a whole lot of time.

But you're right; sometimes, you spend time coming up with your own viable assumption, and it turns out to look nothing like the correct answer. Take a look at the following example:

Thomas watched several videos about how to swing a tennis racket properly. Within several months, his velocity and accuracy were measurably improved. Therefore, the videos must have been the cause.

How would you weaken this argument? I can think of several ways to do so.
1. Thomas started practicing more.
2. Thomas is just getting over an injury.
3. Thomas started receiving lessons.
4. Thomas got some great advice from a playing partner.
There are many more.

The trick here is to think about what these answers all have in common. They all serve the same function. Think about how you would weaken that argument. You would come up with another reason why Thomas improved besides the one the author provides.

So, when pre-phrasing your assumption, don't get too bogged down with the exact assumption you come up with. Rather, focus on what your assumption is accomplishing within the logic of the passage.

Thanks a lot. I will give a try