The effect of media violence on children

Câu hỏi: Read the following passage and mark the letter A, B, C, or D to indicate the correct word or phrase that best fits each the numbered blanks.
Research has shown that the depiction of gun violence is increasing in both movies and on TV. Our research found that acts of gun violence in PG-13 movies has nearly tripled in the 30 years___(39) ___ the rating was introduced in 1984. And PG-13 movies are not exclusively watched by teens and above. A ___(40) ___ of adults in 2019 found that 12% said they were allowed to watch PG-13 movies between the ages of 6 and 9, with 6% saying they watched such films aged even younger.
Although ___(41) ___ skeptics say violent media do not lead children to become more aggressive, a large survey conducted in 2015 found that most pediatricians and media scholars agree that there is a link.
Violent media can also lead children to ___(42) ___ in more dangerous behavior if they find a real gun. In studies one of us conducted, exposure to both movies and video games with guns was found to encourage children ages 8-12 years old to pick up a real gun ___(43) ___ had been hidden in a drawer and pull the trigger, including while pointing the gun at themselves or their friend. This behavior was observed by a hidden camera.
The word or phrase which best fits the gap (40) is...
A. argument
B. experiment
C. survey
D. debate