Assertion: If aluminium atoms replace a few silicon atoms in three-dimensional network of silicon dioxide, the overall structure acquires a negative charge.

Reason: Aluminium is trivalent while silicon is tetravalent.

1. Both assertion and reason are true and the reason is the correct explanation of assertion.

2. Both assertion and reason are true and the reason is not the correct explanation of assertion.

3. Assertion is true but the reason is false.

4. Assertion is false but the reason is true.

Hint: Aluminium outermost shell contains three electrons and silicon contains four electrons.