how to stay focused throughout the whole test?

There are a few things that can help you remain focused while reading longer RC passages:
1. jot down the main points stated in the first few sentences of each paragraph - that way you will have a break from staring at computer screen and also you will have a summary of the whole text
2. try to come up with your own proposal of an answer before you read the answers given
3. skip the parts of the passage with tricky, hard to understand jargon unless the question specifically asks about details from this particular part of the text - difficult vocabulary is supposed to distract you and prevent you from seeing how different parts of the passage are logically connected with each other.