An equation is in function form when it is solved for $y$.  A linear equation is also an equation in which the highest power of the variable is always $1$ 
Which of the following is a linear equation written in function form? 
Options:
A) $x=15$ 
B) $y=5x+7b-120c$ 
C) $y^{2}=4$ 
D) $y=-3x^{2}-1$ 