There are many reasons why Zachary can't concentrate. He might have something on his mind that is bothering him, he could be hungry, he could not have a lot of energy. The best thing Zachary could do to help the paper not suffer is leave his desk and paper alone for about a half an hour, and maybe eat something, or just take a break. Maybe take a nap to get some energy back up so that he can get everything he needs in his paper.
in controlled experiment an independent variable is systematically manupulated and dependent variable is measured ; any extraneous variables are controlled
