Interference pattern

Diagram below shows an interference pattern of two coherent water sources, X and Y.
At which points, A, B, C or D the destructive interference occurred? 
 
Rajah di bawah menunjukkan satu corak interferens terbentuk daripada dua punca
gelombang yang koheren, X dan Y.Pada titik manakah, A, B, C atau D berlakunya
interferens memusnah
 
 

 
 
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Answer
 
B