Find the sum of the expression

$\dfrac{1}{\sqrt{1}+\sqrt{2}}+\dfrac{1}{\sqrt{2}+\sqrt{3}}+\dfrac{1}{\sqrt{3}+\sqrt{4}}+ \dots \dots +\dfrac{1}{\sqrt{80}+\sqrt{81}}$

1. $7$
2. $8$
3. $9$
4. $10$