Which letters are the crests?

a
B & G
b
A & D
c
B & C
d
E & I

Picture would help, but the crests are the high point of a wave

Distance^2= yellow line ^2 (see picture) = 30^2 + 49^2so distance = squareroot of (300^2 +49^2)i dont know if this is how to solve it, but i cnt think of another way