Explanation of the phrase "Good girl/boy!"

[Magellan]

English Language Tests, Intermediate level

ESL/EFL Test #253 "Good, Well and Okay Test (1)", question 7

When you're training a dog, you should always use phrases such as " ......... girl/boy!" to show praise for good behavior.

(a) Good
(b) Well
(c) Okay

English Language Tests, Intermediate level

ESL/EFL Test #253 "Good, Well and Okay Test (1)", answer 7

When you're training a dog, you should always use phrases such as " Good girl/boy!" to show praise for good behavior.

Correct answer: (a) Good

Your answer was: incorrect
When you're training a dog, you should always use phrases such as " Okay girl/boy!" to show praise for good behavior.
_________________________

why?

Magellan

[Lost_Soul]

Just because people talk like that. It's a set expression
I'm afraid you have to just memorize it