The degree of unsaturation, or index of hydrogen deficiency, is the number of pi bonds plus rings in a molecule.

Specify the degree of unsaturation $($index of hydrogen deficiency$)$ of the following formulas:

$($a$)$ C$24$H$24$

$($b$)$ C$3$H$5$F

$($c$)$ C$7$H$7$NO$4$