Answer
Fill the blank with
$A=0$ or $B=0.$
Work Step by Step
Read "AB" as "the product of two numbers, A and B."
If both are nonzero, their product is also nonzero.
Therefore, if the product is zero, one or both of the numbers must be zero (this is the zero-product principle)
Fill the blank with
$A=0$ or $B=0.$
