Phy past paper II 2004 Q 25

Two dippers S1 and S2 are connected to a same vibrator and produce idential waves in a ripple tank. P is a point such that S1P-S2P= wavelength, which of the following statements is/ are correct?

(1) constructive interference occurs at P
(2) a crest is always form at P
(3) if the wavelength of the waves generated by the dippers is doubled, distructive interference will occur at P