which statement is biconditional

a)if x=5, then 2x+3=13
b)if two angles are a linear pair, then they = 180 degrees
c) if the car runs out of gas, then it will stop
d) if a figure is square, then it has four sides

Biconditional means that the first statement is true "if an only if" the second statement is true.

Which alternative fits that definition?