John is testing if cocaine causes increased locomotor activity. He gives 4 rats a dose of cocaine, and another 4 rats a placebo injection. Then he measures locomotor activity and compares the two different groups of rats. What type of design is John using? 

a) Between subjects factorial design 

b) Within subjects factorial design 

c) Between subjects Two-group design 

d) Within-subject Two group design 

e) Between subjects multiple-group design 

f) Within subjects multiple-group design