Professor Martinez Wants to Know If Children Who Receive One-On-One

Professor Martinez wants to know if children who receive one-on-one instruction at school feel more self-confident than children who receive group instruction.To identify a causal relationship between type of instruction and self-confidence,Professor Martinez should

A) compare the number of hours teachers spend on group instruction with the number of hours they spend on individualized instruction.
B) observe a teacher's one-on-one interactions with several children in a laboratory school.
C) select a group of children who score high on the self-confidence measure and a second group who score low on the self-confidence measure and expose them to both types of instruction.
D) randomly assign half of the participants to group instruction and the other half to one-on-one instruction and compare measures of self-confidence for each group.