"The Secret ___," Enid Blyton series about a group of kid sleuths whose mystery-solving club is only open to those who know the password

"Where there's a will, there's a ___"3 Dog's tail response to "Good boy"3 "___ you still with me?"3 "You're the ___ to my yang"3 School pick-up vehicle3 Like a bunch-spoiling apple3 "U" in UV rays5