Which conditional and its converse form a true biconditional?

a) if x> 0, then lxl > 0
b) if x=3, then x2^2=9
c) if x^2 =4, then x=2
d) if x=19, then 2x-3=35

The answer is
D) If x=19, then 2x-3=35
proof

If x=19,  2(19)-3=38-3=35  so  If x=19, then 2x-3=35 is verified
if 2x-3=35, so 2x=35 + 3, 2x=38 implies x = 19, so x =19 is verified

 
If x=19 if and only if  2x-3=35

1/125 probability for each student so, a 0.8 percent probably of winning per student.