Destructive Interference?

Question

A pebble is dropped in a lake and it produces ripples with a frequency of 0.25Hz. When should a second pebble be dropped at the same place to produce destuctive interference?

a. 0.50 s after the first
b. 0.75 s after the first
c. 1.0 s after the first
d. 1.5 s after the first
e. 2.0 s after the first

Thanks :)

Answer 1

e) 2.0 s after the first