Two Studies Were Carried Out to Determine If Children in Daycare

Multiple Choice

Two studies were carried out to determine if children in daycare got more ear infections than children who stayed at home.The first study compared ear infection rates in 50 Canadian girls ages 1 to 2,half of whom attended daycare and half of whom stayed home.The second study compared ear infection rates in 1000 Swedish boys ages 3 to 4,half of whom attended daycare and half of whom stayed home.The results of these two studies differed greatly.
Which of the following is LEAST likely to be the cause of such differences?

A) different genders
B) different mothers
C) different ages
D) different sample sizes
E) different countries