For a three dimensional crystal having n primitive unit cells with a basis of p-atoms , the number of optica branches is:- 1) 3 2) 3p 3) 3p-3 4) 3n-3p

For a three dimensional crystal having N primitive unit cells with a basis of p-atoms , the number of optica branches is:- 1) 3 2) 3p 3) 3p-3 4) 3N-3p