the ¨push pull¨ theory is used by scientists to explain
https://www.google.com/search?q=%C2%A8push+pull%C2%A8+theory&oq=%C2%A8push+pull%C2%A8+theory&aqs=chrome..69i57j69i60.1414j0j7&sourceid=chrome&ie=UTF-8
Read lots and read carefully.
Read lots and read carefully.