Replace largest elements in matrix with NaN

I would like to replace the n largest numbers in first column with NaN in a matrix.

For example for value n = 3 for matrix A output will be B :

A =

17 2

5 68

79 28

900 17

77 63

90 87

B =

17 2

5 68

NaN 28

NaN 17

77 63

NaN 87

What will be the code for this?

Answer 1

[Kevin Phung]

A =[17     2
     5    68
    79    28
   900    17
    77    63
    90    87];

n = 3;
elem = sort(A(:,1),'d');
elem = elem(1:n);
A(ismember(A(:,1),elem)) = NaN;