How can I make elements in a matrix NAN where they don't match that of another matrix?

suppose I have a matrix

A = [1,2,NaN
     4,5,6]

and

B = [12,3,4
     4,5,67]

I want

C = [12,3,NaN
     4,5,67]

How can I go about doing this?

[Wanbin Song]

What do you expect? How does the matrix C comes out? Please explain more

Answer 1

[Walter Roberson]

C = B;
C(isnan(A)) = NaN;