“Opponents of cell phone use in schools believe that cell phones are a distraction to many students.

Read this excerpt from a passage.
“Opponents of cell phone use in schools believe that cell phones are a distraction to many students. They argue that if students have access to personal calls and texts, they will not focus on their classwork. However, it is possible that cell phones can be advantageous for both teachers and students. Educational applications and Internet access on smart phones are useful tools. In history classes, for example, cell phones can be used to explore primary sources on the Internet or to find facts about current events that are not yet in textbooks. Cell phones can also be used to increase student collaboration, critical thinking, and personalized feedback from teachers (Biancarosa & Griffiths, 2012). Cell phones are already enhancing some classroom experiences. According to a Pew Research Center study, 73 percent of Advanced Placement and National Writing Project teachers claim that their students use cell phones to complete assignments in their classrooms (Higgins, 2013).”
Which of these is the attributive phrase that identifies the source of the percentage of Advanced Placement and National Writing Project teachers who allow students to use cell phones to complete assignments?
“According to a Pew Research Center study.”
(Biancarosa & Griffiths, 2012)
“In history classes.”
(Higgins, 2013)