4.Although the most important American baseball league - Major League Baseball - strives to ban players from chewing tobacco, most of them continue to do it. A team of researches wants to determine the relation between tobacco chewing and the occurrence of contact mucositis of the oral cavity among players. At a training camp they ask players about their tobacco chewing, smoking and alcohol drinking. They also estimate the condition of the players’ oral cavity mucosa. Which of the following the researches carry out?