Computer Science Principles of Programming Languages.

Consider the following grammar:

$S$::$=AB$

$A$::$=aA|a$

$B$::$=b$

and the following derivation:

$S=>AB$

$=>$aAB

$=>$aAb

$=>$aab

Which of the following are true?

This is a leftmost derivation

This is a rightmost derivation

This is not a valid derivation

This is a valid derivation, but it is not leftmost nor rightmost