A researcher wants to determine if extra homework problems help 8th grade students learn algebra. An 8th grade class is divided into pairs and one student from each pair has extra homework problems and the other in the pair does not. After 2 weeks, the entire class takes an algebra test and the results of the two groups are compared. To be a valid matched pair test, what should the researcher consider in creating the two groups?

(a) That each pair of students has similar IQs or abilities in mathematics
(b) That the group with the extra homework problems has fewer after school activities
(c) That the group without extra homework problems receives different instruction
(d) That each pair of students has similar ages at the time of the testing