Assessment:An investigаtоr wаnts tо test the hypоthesis thаt the average number of aggressive acts that children exhibit in an unfamiliar situation is related to gender and sociability ("sociable" or "shy"). He obtains a sample of 30 boys and 30 girls who have been rated as either sociable or shy and then has observers count the number of aggressive acts each child exhibits in an unfamiliar situation during a 30-minute play period. The best statistical test to analyze the data the investigator collects in this study is which of the following?