Achieve Week 8 question

[Joseph Dekel 1F]

The last question of Achieve Week 8 Homework asks if an (AsO4)^3- ion is polar or non-polar and the correct answer is "polar". I understand that it has a tetrahedral shape which is symmetrical, but it literally has a charge of -3 which is built up on the oxygens with the single bond, so there is a clear dipole meaning that it is polar. Could somebody please explain why this explanation is wrong and why the correct answer is "polar"?

[Alejandro_Franco_3B]

The (AsO4)^3- ion is non-polar. The dipoles of polar bonds (As-O) cancel out because of the symmetrical tetrahedral shape of the molecule.